PERLRETUT(1)
💖 NAME 💖
perlretut - Perl regular expressions tutorial ✨ yep bestie, this is perlretut(1) and it's here to make regex make sense.
📖 DESCRIPTION 📖
This page is your basic tutorial on understanding, creating, and using regular expressions in Perl. It's the cozy companion to the reference page on regular expressions, perlre. Regular expressions are an integral part of the m//, s///, qr// and split operators, so this tutorial also overlaps with "Regexp Quote-Like Operators" in perlop and "split" in perlfunc.
Perl is widely renowned for excellence in text processing, and regular expressions are one of the big factors behind this fame. Perl regular expressions display an efficiency and flexibility unknown in most other computer languages. Mastering even the basics of regular expressions will allow you to manipulate text with surprising ease. slay 💅
So what even is a regular expression? A regular expression is simply a string that describes a pattern. Patterns are in common use these days; examples are the patterns typed into a search engine to find web pages and the patterns used to list files in a directory, e.g., ls *.txt or dir *.*. In Perl, the patterns described by regular expressions are used to search strings, extract desired parts of strings, and to do search and replace operations.
Regular expressions have the undeserved reputation of being abstract and difficult to understand. Not me defending regex at 2 a.m. Regular expressions are constructed using simple concepts like conditionals and loops and are no more difficult to understand than the corresponding if conditionals and while loops in the Perl language itself. In fact, the main challenge in learning regular expressions is just getting used to the terse notation used to express these concepts.
This tutorial flattens the learning curve by discussing regular expression concepts, along with their notation, one at a time and with many examples. The first part of the tutorial will progress from the simplest word searches to the basic regular expression concepts. If you master the first part, you will have all the tools needed to solve about 98% of your needs. The second part of the tutorial is for those comfortable with the basics and hungry for more power tools. It discusses the more advanced regular expression operators and introduces the latest cutting-edge innovations. 🚀
A note: to save time, 'regular expression' is often abbreviated as regexp or regex. Regexp is a more natural abbreviation than regex, but is harder to pronounce. The Perl pod documentation is evenly split on regexp vs regex; in Perl, there is more than one way to abbreviate it. We'll use regexp in this tutorial. iykyk ✨
🌈 Part 1: The basics 🌈
🎀 Simple word matching 🎀
The simplest regexp is simply a word, or more generally, a string of characters. A regexp consisting of a word matches any string that contains that word:
"Hello World" =~ /World/; # matchesOkay so what is this Perl statement all about? "Hello World" is a simple double-quoted string. "World" is the regular expression and the // enclosing /World/ tells Perl to search a string for a match. The operator =~ associates the string with the regexp match and produces a true value if the regexp matched, or false if the regexp did not match. In our case, "World" matches the second word in "Hello World", so the expression is true. Expressions like this are useful in conditionals:
if ("Hello World" =~ /World/) {
print "It matches\n";
}
else {
print "It doesn't match\n";
}There are useful variations on this theme. The sense of the match can be reversed by using the !~ operator:
if ("Hello World" !~ /World/) {
print "It doesn't match\n";
}
else {
print "It matches\n";
}The literal string in the regexp can be replaced by a variable:
$greeting = "World";
if ("Hello World" =~ /$greeting/) {
print "It matches\n";
}
else {
print "It doesn't match\n";
}If you're matching against the special default variable $_, the $_ =~ part can be omitted:
$_ = "Hello World";
if (/World/) {
print "It matches\n";
}
else {
print "It doesn't match\n";
}And finally, the // default delimiters for a match can be changed to arbitrary delimiters by putting an 'm' out front:
"Hello World" =~ m!World!; # matches, delimited by '!'
"Hello World" =~ m{World}; # matches, note the matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
# '/' becomes an ordinary char/World/, m!World!, and m{World} all represent the same thing. When, e.g., the quote (") is used as a delimiter, the forward slash / becomes an ordinary character and can be used in this regexp without trouble.
Let's consider how different regexps would match "Hello World":
"Hello World" =~ /world/; # doesn't match
"Hello World" =~ /o W/; # matches
"Hello World" =~ /oW/; # doesn't match
"Hello World" =~ /World /; # doesn't matchThe first regexp world doesn't match because regexps are case-sensitive. The second regexp matches because the substring 'o W' occurs in the string "Hello World". The space character ' ' is treated like any other character in a regexp and is needed to match in this case. The lack of a space character is the reason the third regexp 'oW' doesn't match. The fourth regexp 'World ' doesn't match because there is a space at the end of the regexp, but not at the end of the string. The lesson here is that regexps must match a part of the string exactly in order for the statement to be true.
If a regexp matches in more than one place in the string, Perl will always match at the earliest possible point in the string:
"Hello World" =~ /o/; # matches 'o' in 'Hello'
"That hat is red" =~ /hat/; # matches 'hat' in 'That'With respect to character matching, there are a few more points you need to know about. First of all, not all characters can be used 'as is' in a match. Some characters, called metacharacters, are reserved for use in regexp notation. The metacharacters are
{}[]()^$.|*+?\The significance of each of these will be explained in the rest of the tutorial, but for now, it is important only to know that a metacharacter can be matched by putting a backslash before it:
"2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
"2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
"The interval is [0,1)." =~ /[0,1)./ # is a syntax error!
"The interval is [0,1)." =~ /\[0,1\)\./ # matches
"#!/usr/bin/perl" =~ /#!\/usr\/bin\/perl/; # matchesIn the last regexp, the forward slash / is also backslashed, because it is used to delimit the regexp. This can lead to LTS (leaning toothpick syndrome), however, and it is often more readable to change delimiters.
"#!/usr/bin/perl" =~ m!#\!/usr/bin/perl!; # easier to readThe backslash character '\' is a metacharacter itself and needs to be backslashed:
'C:\WIN32' =~ /C:\\WIN/; # matchesIn addition to the metacharacters, there are some ASCII characters which don't have printable character equivalents and are instead represented by escape sequences. Common examples are "\t" for a tab, "\n" for a newline, "\r" for a carriage return and "\a" for a bell (or alert). If your string is better thought of as a sequence of arbitrary bytes, the octal escape sequence, e.g., "\033", or hexadecimal escape sequence, e.g., "\x1B" may be a more natural representation for your bytes. Here are some examples of escapes:
"1000\t2000" =~ m(0\t2) # matches
"1000\n2000" =~ /0\n20/ # matches
"1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000"
"cat" =~ /\o{143}\x61\x74/ # matches in ASCII, but a weird way
# to spell catIf you've been around Perl a while, all this talk of escape sequences may seem familiar. Similar escape sequences are used in double-quoted strings and in fact the regexps in Perl are mostly treated as double-quoted strings. This means that variables can be used in regexps as well. Just like double-quoted strings, the values of the variables in the regexp will be substituted in before the regexp is evaluated for matching purposes. So we have:
$foo = 'house';
'housecat' =~ /$foo/; # matches
'cathouse' =~ /cat$foo/; # matches
'housecat' =~ /${foo}cat/; # matchesSo far, so good. With the knowledge above you can already perform searches with just about any literal string regexp you can dream up. Here is a very simple emulation of the Unix grep program:
% cat > simple_grep
#!/usr/bin/perl
$regexp = shift;
while (<>) {
print if /$regexp/;
}
^D% chmod +x simple_grep% simple_grep abba /usr/dict/words
Babbage
cabbage
cabbages
sabbath
Sabbathize
Sabbathizes
sabbatical
scabbard
scabbardsThis program is easy to understand. #!/usr/bin/perl is the standard way to invoke a perl program from the shell. $regexp = shift; saves the first command line argument as the regexp to be used, leaving the rest of the command line arguments to be treated as files. while (<>) loops over all the lines in all the files. For each line, print if /$regexp/; prints the line if the regexp matches the line. In this line, both print and /$regexp/ use the default variable $_ implicitly.
With all of the regexps above, if the regexp matched anywhere in the string, it was considered a match. Sometimes, however, we'd like to specify where in the string the regexp should try to match. To do this, we would use the anchor metacharacters ^ and $. The anchor ^ means match at the beginning of the string and the anchor $ means match at the end of the string, or before a newline at the end of the string. Here is how they are used:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /^keeper/; # doesn't match
"housekeeper" =~ /keeper$/; # matches
"housekeeper\n" =~ /keeper$/; # matchesThe second regexp doesn't match because ^ constrains keeper to match only at the beginning of the string, but "housekeeper" has keeper starting in the middle. The third regexp does match, since the $ constrains keeper to match only at the end of the string.
When both ^ and $ are used at the same time, the regexp has to match both the beginning and the end of the string, i.e., the regexp matches the whole string. Consider
"keeper" =~ /^keep$/; # doesn't match
"keeper" =~ /^keeper$/; # matches
"" =~ /^$/; # ^$ matches an empty stringSo the first regexp is a no because the string has more going on than just "keep". The second one is literally the whole string, so it matches, slay. Using both "^" and "$" in a regexp forces the complete string to match, which gives you full control over which strings match and which ones don't. Say you're looking for a fellow named bert, all by himself in a string:
"dogbert" =~ /bert/; # matches, but not what you want
"dilbert" =~ /^bert/; # doesn't match, but ..
"bertram" =~ /^bert/; # matches, so still not good enough
"bertram" =~ /^bert$/; # doesn't match, good
"dilbert" =~ /^bert$/; # doesn't match, good
"bert" =~ /^bert$/; # matches, perfectObviously, if it's just a literal string, you could use the string comparison "$string eq 'bert'" and that would be more efficient. The "^...$" regexp really starts earning its keep once we add the more powerful regexp tools below.
✨ Using character classes ✨
Okay so literal string regexps already do a lot, but bestie, we've only scratched the surface of regular expression technology. In this section and the next ones, we're introducing regexp concepts (and the associated metacharacter notations) that let a regexp represent not just one single character sequence, but a whole class of them.
One of those concepts is the character class. A character class lets a set of possible characters — not just a single character — match at a particular point in a regexp. Character classes are written with brackets "[...]", with the set of characters that might match sitting inside. Examples:
/cat/; # matches 'cat'
/[bcr]at/; # matches 'bat, 'cat', or 'rat'
/item[0123456789]/; # matches 'item0' or ... or 'item9'
"abc" =~ /[cab]/; # matches 'a'In that last statement, even though 'c' is the first character in the class, 'a' matches because the first character position in the string is the earliest point where the regexp can match. Iconic.
/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.This regexp is doing a super common task: a case-insensitive match. Perl gives you a way to avoid all those brackets by just appending an 'i' to the end of the match. So "/[yY][eE][sS]/;" can be rewritten as "/yes/i;". The 'i' stands for case-insensitive and is an example of a modifier of the matching operation. We'll meet other modifiers later in the tutorial, don't worry.
We saw above that there were ordinary characters, which represented themselves, and special characters, which needed a backslash "\" to represent themselves. Same deal inside a character class, but the sets of ordinary and special characters inside a character class are different from those outside one. The special characters for a character class are "-]\^$" (and the pattern delimiter, whatever it is). "]" is special because it means the end of a character class. "$" is special because it means a scalar variable. "\" is special because it's used in escape sequences, just like above. Here's how the special characters "]$\" are handled:
/[\]c]def/; # matches ']def' or 'cdef'
$x = 'bcr';
/[$x]at/; # matches 'bat', 'cat', or 'rat'
/[\$x]at/; # matches '$at' or 'xat'
/[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'The last two are a little tricky. In "[\$x]", the backslash protects the dollar sign, so the character class has two members: "$" and "x". In "[\\$x]", the backslash itself is protected, so $x is treated as a variable and substituted in double quote fashion. Obsessed with the drama of escaping, honestly.
The special character '-' acts as a range operator inside character classes, so a contiguous set of characters can be written as a range. With ranges, the unwieldy "[0123456789]" and "[abc...xyz]" become the svelte "[0-9]" and "[a-z]". Some examples:
/item[0-9]/; # matches 'item0' or ... or 'item9'
/[0-9bx-z]aa/; # matches '0aa', ..., '9aa',
# 'baa', 'xaa', 'yaa', or 'zaa'
/[0-9a-fA-F]/; # matches a hexadecimal digit
/[0-9a-zA-Z_]/; # matches a "word" character,
# like those in a Perl variable nameIf '-' is the first or last character in a character class, it's treated as an ordinary character; "[-ab]", "[ab-]" and "[a\-b]" are all equivalent. Cute.
The special character "^" in the first position of a character class means a negated character class, which matches any character but the ones in the brackets. Both "[...]" and "[^...]" must match a character, or the match fails. Then:
/[^a]at/; # doesn't match 'aat' or 'at', but matches
# all other 'bat', 'cat, '0at', '%at', etc.
/[^0-9]/; # matches a non-numeric character
/[a^]at/; # matches 'aat' or '^at'; here '^' is ordinaryNow, even "[0-9]" can be a bother to write multiple times, so to save keystrokes and make regexps more readable, Perl has several abbreviations for common character classes, as shown below. Since Unicode showed up, unless the "//a" modifier is in effect, these character classes match more than just a few characters in the ASCII range.
\d— matches a digit, not just[0-9]but also digits from non-roman scripts\s— matches a whitespace character, the set[\ \t\r\n\f]and others\w— matches a word character (alphanumeric or_), not just[0-9a-zA-Z_]but also digits and characters from non-roman scripts\D— is a negated\d; it represents any other character than a digit, or[^\d]\S— is a negated\s; it represents any non-whitespace character[^\s]\W— is a negated\w; it represents any non-word character[^\w]- The period
'.'— matches any character but"\n"(unless the modifier"//s"is in effect, as explained below). \N— like the period, matches any character but"\n", but it does so regardless of whether the modifier"//s"is in effect.
The "//a" modifier, available starting in Perl 5.14, is used to restrict the matches of \d, \s, and \w to just those in the ASCII range. It's useful for keeping your program from being needlessly exposed to full Unicode (and its accompanying security considerations) when all you want is to process English-like text. (The "a" may be doubled, "//aa", to provide even more restrictions, preventing case-insensitive matching of ASCII with non-ASCII characters; otherwise a Unicode "Kelvin Sign" would caselessly match a "k" or "K".) The way Unicode just sneaks in? Lowkey wild.
The "\d\s\w\D\S\W" abbreviations can be used both inside and outside of character classes. Here are some in use:
/\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
/[\d\s]/; # matches any digit or whitespace character
/\w\W\w/; # matches a word char, followed by a
# non-word char, followed by a word char
/..rt/; # matches any two chars, followed by 'rt'
/end\./; # matches 'end.'
/end[.]/; # same thing, matches 'end.'Because a period is a metacharacter, it needs to be escaped to match as an ordinary period. And because, for example, "\d" and "\w" are sets of characters, it is incorrect to think of "[^\d\w]" as "[\D\W]"; in fact "[^\d\w]" is the same as "[^\w]", which is the same as "[\W]". Think DeMorgan's laws, bestie.
An anchor that's useful in basic regexps is the word anchor "\b". This matches a boundary between a word character and a non-word character "\w\W" or "\W\w":
$x = "Housecat catenates house and cat";
$x =~ /cat/; # matches cat in 'housecat'
$x =~ /\bcat/; # matches cat in 'catenates'
$x =~ /cat\b/; # matches cat in 'housecat'
$x =~ /\bcat\b/; # matches 'cat' at end of stringNote that in the last example, the end of the string is considered a word boundary. Periodt.
You might wonder why '.' matches everything but "\n" — like, why not every character? The reason is that often you're matching against lines and you'd like to ignore the newline characters. For instance, while the string "\n" represents one line, we'd like to think of it as empty. Then:
"" =~ /^$/; # matches
"\n" =~ /^$/; # matches, $ anchors before "\n"
"" =~ /./; # doesn't match; it needs a char
"" =~ /^.$/; # doesn't match; it needs a char
"\n" =~ /^.$/; # doesn't match; it needs a char other than "\n"
"a" =~ /^.$/; # matches
"a\n" =~ /^.$/; # matches, $ anchors before "\n"This behavior is convenient, because we usually want to ignore newlines when we count and match characters in a line. Sometimes, though, we want to keep track of newlines. We might even want "^" and "$" to anchor at the beginning and end of lines within the string, rather than just the beginning and end of the string. Perl lets us choose between ignoring and paying attention to newlines by using the "//s" and "//m" modifiers. "//s" and "//m" stand for single line and multi-line, and they determine whether a string is treated as one continuous string or as a set of lines. These two modifiers affect two aspects of how the regexp is interpreted: 1) how the '.' character class is defined, and 2) where the anchors "^" and "$" are able to match. Here are the four possible combinations:
- no modifiers (
//): Default behavior.'.'matches any character except"\n"."^"matches only at the beginning of the string and"$"matches only at the end or before a newline at the end. smodifier (//s): Treat string as a single long line.'.'matches any character, even"\n"."^"matches only at the beginning of the string and"$"matches only at the end or before a newline at the end.mmodifier (//m): Treat string as a set of multiple lines.'.'matches any character except"\n"."^"and"$"are able to match at the start or end of any line within the string.- both
sandmmodifiers (//sm): Treat string as a single long line, but detect multiple lines.'.'matches any character, even"\n"."^"and"$", however, are able to match at the start or end of any line within the string.
Here are examples of "//s" and "//m" in action:
$x = "There once was a girl\nWho programmed in Perl\n";
$x =~ /^Who/; # doesn't match, "Who" not at start of string
$x =~ /^Who/s; # doesn't match, "Who" not at start of string
$x =~ /^Who/m; # matches, "Who" at start of second line
$x =~ /^Who/sm; # matches, "Who" at start of second lineSo peep these four lines and clock which ones actually hit:
$x =~ /girl.Who/; # doesn't match, "." doesn't match "\n"
$x =~ /girl.Who/s; # matches, "." matches "\n"
$x =~ /girl.Who/m; # doesn't match, "." doesn't match "\n"
$x =~ /girl.Who/sm; # matches, "." matches "\n"<tbh>most of the time the default behaviour is literally exactly what you want</tbh>, but //s and //m are occasionally very useful. And here's the slay part — if you're running //m, the start of the string can still be matched with \A, and the end of the string can still be matched with the anchors \Z (which matches both the end and the newline before it, just like $) and \z (which matches only the end). Periodt:
$x =~ /^Who/m; # matches, "Who" at start of second line
$x =~ /\AWho/m; # doesn't match, "Who" is not at start of string
$x =~ /girl$/m; # matches, "girl" at end of first line
$x =~ /girl\Z/m; # doesn't match, "girl" is not at end of string
$x =~ /Perl\Z/m; # matches, "Perl" is at newline before end
$x =~ /Perl\z/m; # doesn't match, "Perl" is not at end of stringSo now we know how to create choices among classes of characters in a regexp. But what about choices among whole words or character strings? That's giving next-section energy, bestie — it's coming right up. 🦋
✨ Matching this or that ✨
Sometimes we want our regexp to match different possible words or character strings, and that's exactly what the alternation metacharacter | is for. Wanna match dog or cat? You form the regexp dog|cat. 🐶🐱
As before, Perl tries to match the regexp at the earliest possible point in the string. At each character position, Perl first tries to match the first alternative, dog. If dog doesn't match, Perl tries the next alternative, cat. If cat doesn't match either, the match fails and Perl moves on to the next position in the string. Some examples, because examples are the moment:
"cats and dogs" =~ /cat|dog|bird/; # matches "cat"
"cats and dogs" =~ /dog|cat|bird/; # matches "cat"Even though dog is the first alternative in the second regexp, cat is able to match earlier in the string. Leftmost wins, always. 💅
"cats" =~ /c|ca|cat|cats/; # matches "c"
"cats" =~ /cats|cat|ca|c/; # matches "cats"Here, all the alternatives match at the first string position, so the first alternative is the one that matches. So if some of your alternatives are truncations of the others, put the longest ones first so they actually get a chance to match. Iconic advice, fr fr.
"cab" =~ /a|b|c/ # matches "c"
# /a|b|c/ == /[abc]/The last example is a little reveal: character classes are basically alternations of characters. At a given character position, the first alternative that lets the regexp match succeed is the one that matches. Simple as that. 💫
🎀 Grouping things and hierarchical matching 🎀
Alternation lets a regexp choose among alternatives, but on its own it's kind of unsatisfying. Why? Because each alternative is a whole regexp, and sometimes we only want alternatives for part of a regexp.
Say we want to search for housecat or housekeeper. The regexp housecat|housekeeper fits the bill, but it's inefficient because we had to type house twice 😭. It would be so much nicer to have parts of the regexp be constant, like house, and some parts have alternatives, like cat|keeper.
Enter the grouping metacharacters (). 🎉 Grouping lets parts of a regexp be treated as a single unit — you group parts of a regexp by enclosing them in parentheses. So we could solve housecat|housekeeper by forming the regexp as house(cat|keeper). That regexp means: match house followed by either cat or keeper. Some more examples are
/(a|b)b/; # matches 'ab' or 'bb'
/(ac|b)b/; # matches 'acb' or 'bb'
/(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
/(a|[bc])d/; # matches 'ad', 'bd', or 'cd'
/house(cat|)/; # matches either 'housecat' or 'house'
/house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
# 'house'. Note groups can be nested.
/(19|20|)\d\d/; # match years 19xx, 20xx, or the Y2K problem, xx
"20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
# because '20\d\d' can't matchAlternations behave the same way inside groups as outside them: at a given string position, the leftmost alternative that allows the regexp to match is the one taken. So in that last example, at the first string position, 20 matches the second alternative — but there's nothing left over to match the next two digits \d\d. 😬 So Perl moves on to the next alternative, which is the null alternative, and that one works, since 20 is two digits. Slay.
🧚 backtracking, a walk in the woods 🧚
The process of trying one alternative, seeing if it matches, and moving on to the next alternative — while going back in the string to where the previous alternative was tried, if it didn't — is called backtracking.
The term comes from the idea that matching a regexp is like a walk in the woods. 🌲 Successfully matching a regexp is like arriving at a destination. There are many possible trailheads, one for each string position, and each one is tried in order, left to right. From each trailhead there may be many paths — some get you there, some are dead ends. When you walk along a trail and hit a dead end, you have to backtrack along the trail to an earlier point to try another trail. If you hit your destination, you stop immediately and forget about trying all the other trails. You are persistent, bestie, and only if you've tried all the trails from all the trailheads and not arrived at your destination do you declare failure.
To be concrete, here's a step-by-step analysis of what Perl does when it tries to match the regexp
"abcde" =~ /(abd|abc)(df|d|de)/;- Start with the first letter in the string
'a'. - Try the first alternative in the first group
'abd'. - Match
'a'followed by'b'. So far so good. 'd'in the regexp doesn't match'c'in the string — a dead end. So backtrack two characters and pick the second alternative in the first group'abc'.- Match
'a'followed by'b'followed by'c'. We are on a roll and have satisfied the first group. Set$1to'abc'. - Move on to the second group and pick the first alternative
'df'. - Match the
'd'. 'f'in the regexp doesn't match'e'in the string, so a dead end. Backtrack one character and pick the second alternative in the second group'd'.'d'matches. The second grouping is satisfied, so set$2to'd'.- We are at the end of the regexp, so we are done! We have matched
'abcd'out of the string"abcde".
There are a couple of things to note about this analysis. 🫶 First, the third alternative in the second group 'de' also allows a match, but we stopped before we got to it — at a given character position, leftmost wins. Second, we were able to get a match at the first character position of the string 'a'. If there were no matches at the first position, Perl would move to the second character position 'b' and attempt the match all over again. Only when all possible paths at all possible character positions have been exhausted does Perl give up and declare "$string =~ /(abd|abc)(df|d|de)/;" to be false.
Even with all this work, regexp matching happens remarkably fast. To speed things up, Perl compiles the regexp into a compact sequence of opcodes that can often fit inside a processor cache. When the code is executed, these opcodes can then run at full throttle and search very quickly. 🔥
💜 Extracting matches 💜
The grouping metacharacters () also serve a completely different function: they let you extract the parts of a string that matched. This is so useful for finding out what matched and for text processing in general. For each grouping, the part that matched inside goes into the special variables $1, $2, etc. — and you can use them just like ordinary variables:
# extract hours, minutes, seconds
if ($time =~ /(\d\d):(\d\d):(\d\d)/) { # match hh:mm:ss format
$hours = $1;
$minutes = $2;
$seconds = $3;
}Now, we know that in scalar context, $time =~ /(\d\d):(\d\d):(\d\d)/ returns a true or false value. In list context, though, it returns the list of matched values ($1,$2,$3). So we could write the code more compactly as
# extract hours, minutes, seconds
($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);If the groupings in a regexp are nested, $1 gets the group with the leftmost opening parenthesis, $2 the next opening parenthesis, etc. Here's a regexp with nested groups:
/(ab(cd|ef)((gi)|j))/;
1 2 34If this regexp matches, $1 contains a string starting with 'ab', $2 is either set to 'cd' or 'ef', $3 equals either 'gi' or 'j', and $4 is either set to 'gi', just like $3, or it remains undefined.
For convenience, Perl sets $+ to the string held by the highest numbered $1, $2,... that got assigned (and, somewhat related, $^N to the value of the $1, $2,... most-recently assigned; i.e. the $1, $2,... associated with the rightmost closing parenthesis used in the match). Little treats. 🍓
🌈 Backreferences 🌈
Closely associated with the matching variables $1, $2, ... are the backreferences \g1, \g2,... Backreferences are simply matching variables that can be used inside a regexp. This is a genuinely nice feature — what matches later in a regexp is made to depend on what matched earlier in the regexp. 🤯
Say we wanted to look for doubled words in a text, like 'the the'. The following regexp finds all 3-letter doubles with a space in between:
/\b(\w\w\w)\s\g1\b/;The grouping assigns a value to \g1, so that the same 3-letter sequence is used for both parts. Chef's kiss.
A similar task is to find words consisting of two identical parts:
% simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\g1$' /usr/dict/words
beriberi
booboo
coco
mama
murmur
papaThe regexp has a single grouping which considers 4-letter combinations, then 3-letter combinations, etc., and uses \g1 to look for a repeat.
⚠️ okay so this is important: although$1and\g1represent the same thing, care should be taken to use matched variables$1,$2,... only outside a regexp and backreferences\g1,\g2,... only inside a regexp. Not doing so may lead to surprising and unsatisfactory results. Don't do it, bestie. 🙈
⭐ Relative backreferences ⭐
Counting the opening parentheses to get the correct number for a backreference is error-prone as soon as there's more than one capturing group. A more convenient technique became available with Perl 5.10: relative backreferences. To refer to the immediately preceding capture group you can now write \g{-1}, the next but last is available via \g{-2}, and so on. The way that just... works? Obsessed. 👑
Okay so, another reason besides readability and maintainability to use relative backreferences? Let this example show you, where a cute little pattern matches weird strings:
$a99a = '([a-z])(\d)\g2\g1'; # matches a11a, g22g, x33x, etc.So now we've got this pattern chillin' as a handy string, and you're like "omg let me just drop it into another pattern":
$line = "code=e99e";
if ($line =~ /^(\w+)=$a99a$/){ # unexpected behavior!
print "$1 is valid\n";
} else {
print "bad line: '$line'\n";
}But nope, this doesn't match — at least not the way you'd expect. 😭 Only after you interpolate $a99a and actually stare at the full resulting regexp text does it become obvious that the backreferences have backfired on you. The subexpression (\w+) straight up snatched number 1 and demoted the groups in $a99a by one rank. Rude. Avoid this whole mess with relative backreferences:
$a99a = '([a-z])(\d)\g{-1}\g{-2}'; # safe for being interpolatedNamed backreferences 🏷️
Perl 5.10 also brought us named capture groups and named backreferences, and honestly? Iconic. To attach a name to a capturing group you write either (?<name>...) or (?'name'...). Then the backreference can be written as \g{name}. You're allowed to slap the same name on more than one group, but bestie, in that case only the leftmost one of the eponymous set can be referenced. Outside of the pattern a named capture group is accessible through the %+ hash. 💖
Say we've got to match calendar dates that might come in one of three formats — yyyy-mm-dd, mm/dd/yyyy or dd.mm.yyyy. We can write three suitable patterns where we use 'd', 'm' and 'y' respectively as the names of the groups capturing the pertaining components of a date. The matching operation combines the three patterns as alternatives:
$fmt1 = '(?<y>\d\d\d\d)-(?<m>\d\d)-(?<d>\d\d)';
$fmt2 = '(?<m>\d\d)/(?<d>\d\d)/(?<y>\d\d\d\d)';
$fmt3 = '(?<d>\d\d)\.(?<m>\d\d)\.(?<y>\d\d\d\d)';
for my $d qw( 2006-10-21 15.01.2007 10/31/2005 ){
if ( $d =~ m{$fmt1|$fmt2|$fmt3} ){
print "day=$+{d} month=$+{m} year=$+{y}\n";
}
}If any of the alternatives matches, the hash %+ is bound to contain the three key-value pairs. Slay. ✨
Alternative capture group numbering 🔢
Yet another capturing group numbering technique (also as from Perl 5.10) deals with the problem of referring to groups within a set of alternatives. Consider a pattern for matching a time of the day, civil or military style:
if ( $time =~ /(\d\d|\d):(\d\d)|(\d\d)(\d\d)/ ){
# process hour and minute
}Processing the results requires an extra if statement to figure out whether $1 and $2 or $3 and $4 contain the goodies. Annoying. It would be easier if we could use group numbers 1 and 2 in the second alternative as well, and that's exactly what the parenthesized construct (?|...), set around an alternative, achieves. Here's an extended version of the previous pattern:
if ( $time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/ ){
print "hour=$1 minute=$2 zone=$3\n";
}Within the alternative numbering group, group numbers start at the same position for each alternative. After the group, numbering continues with one higher than the maximum reached across all the alternatives. No cap, that's just how it works. 💅
Position information 📍
In addition to what was matched, Perl (since 5.6.0) also hands you the positions of what was matched as contents of the @- and @+ arrays. $-[0] is the position of the start of the entire match and $+[0] is the position of the end. Similarly, $-[n] is the position of the start of the $n match and $+[n] is the position of the end. If $n is undefined, so are $-[n] and $+[n]. So then this code
$x = "Mmm...donut, thought Homer";
$x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
foreach $expr (1..$#-) {
print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n";
}prints
Match 1: 'Mmm' at position (0,3)
Match 2: 'donut' at position (6,11)Even if there are no groupings in a regexp, it's still possible to find out what exactly matched in a string. If you use them, Perl will set $` to the part of the string before the match, will set $& to the part of the string that matched, and will set $' to the part of the string after the match. Example time:
$x = "the cat caught the mouse";
$x =~ /cat/; # $` = 'the ', $& = 'cat', $' = ' caught the mouse'
$x =~ /the/; # $` = '', $& = 'the', $' = ' cat caught the mouse'In the second match, $` equals '' because the regexp matched at the first character position in the string and stopped; it never saw the second 'the'. 🫶
⚠️ Important note, bestie: using$`and$'slows down regexp matching quite a bit, while$&slows it down to a lesser extent, because if they're used in one regexp in a program, they get generated for all regexps in the program. So if raw performance is a goal of your application, avoid them. If you need to extract the corresponding substrings, use@-and@+instead:
$` is the same as substr( $x, 0, $-[0] )
$& is the same as substr( $x, $-[0], $+[0]-$-[0] )
$' is the same as substr( $x, $+[0] )As of Perl 5.10, the ${^PREMATCH}, ${^MATCH} and ${^POSTMATCH} variables may be used. These are only set if the /p modifier is present. Consequently they do not penalize the rest of the program. Obsessed with a lowkey considerate design choice. 🎀
Non-capturing groupings 🙈
A group that's required to bundle a set of alternatives may or may not be useful as a capturing group. If it isn't, it just creates a superfluous addition to the set of available capture group values, inside as well as outside the regexp. Non-capturing groupings, denoted by (?:regexp), still let the regexp be treated as a single unit, but don't establish a capturing group at the same time. Both capturing and non-capturing groupings are allowed to co-exist in the same regexp. And because there's no extraction, non-capturing groupings are faster than capturing groupings. 🔥
Non-capturing groupings are also super handy for choosing exactly which parts of a regexp get extracted to matching variables:
# match a number, $1-$4 are set, but we only want $1
/([+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?)/;
# match a number faster , only $1 is set
/([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?)/;
# match a number, get $1 = whole number, $2 = exponent
/([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE]([+-]?\d+))?)/;They're also useful for removing nuisance elements gathered from a split operation where parentheses are required for some reason:
$x = '12aba34ba5';
@num = split /(a|b)+/, $x; # @num = ('12','a','34','a','5')
@num = split /(?:a|b)+/, $x; # @num = ('12','34','5')See how much cleaner that second one is? Chef's kiss. 😌
Matching repetitions 🔁
The examples in the previous section display an annoying weakness. We were only matching 3-letter words, or chunks of words of 4 letters or less. We'd like to match words — or, more generally, strings of any length — without writing out tedious alternatives like \w\w\w\w|\w\w\w|\w\w|\w. Literally nobody has time for that.
This is exactly the problem the quantifier metacharacters ?, *, +, and {} were created for. They let us delimit the number of repeats for a portion of a regexp we consider to be a match. Quantifiers go immediately after the character, character class, or grouping that we want to specify. Their meanings, bestie:
a?— match'a'1 or 0 timesa*— match'a'0 or more times, i.e., any number of timesa+— match'a'1 or more times, i.e., at least oncea{n,m}— match at leastntimes, but not more thanmtimesa{n,}— match at leastnor more timesa{n}— match exactlyntimes
Here are some examples:
/[a-z]+\s+\d*/; # match a lowercase word, at least one space, and
# any number of digits
/(\w+)\s+\g1/; # match doubled words of arbitrary length
/y(es)?/i; # matches 'y', 'Y', or a case-insensitive 'yes'
$year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more
# than 4 digits
$year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3-digit dates
$year =~ /^\d{2}(\d{2})?$/; # same thing written differently. However,
# this captures the last two digits in $1
# and the other does not.
% simple_grep '^(\w+)\g1$' /usr/dict/words # isn't this easier?
beriberi
booboo
coco
mama
murmur
papaFor all of these quantifiers, Perl will try to match as much of the string as possible, while still allowing the regexp to succeed. So with /a?.../, Perl will first try to match the regexp with the a present; if that fails, Perl will try to match the regexp without the a present. For the quantifier *, we get the following:
$x = "the cat in the hat";
$x =~ /^(.*)(cat)(.*)$/; # matches,
# $1 = 'the '
# $2 = 'cat'
# $3 = ' in the hat'Which is what we might expect — the match finds the only cat in the string and locks onto it. But consider, however, this regexp:
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 characters match)You might initially guess that Perl would find the at in cat and stop there, but that wouldn't give the longest possible string to the first quantifier .*. Nope! Instead, the first quantifier .* grabs as much of the string as possible while still having the regexp match. In this example, that means having the at sequence with the final at in the string. The other important principle illustrated here is that, when there are two or more elements in a regexp, the leftmost quantifier, if there is one, gets to grab as much of the string as possible, leaving the rest of the regexp to fight over scraps. So in our example, the first quantifier .* grabs most of the string, while the second quantifier .* gets the empty string. Quantifiers that grab as much of the string as possible are called maximal match or greedy quantifiers. Main character energy, honestly. 👑
When a regexp can match a string in several different ways, we can use the principles above to predict which way the regexp will match:
- Principle 0: Taken as a whole, any regexp will be matched at the earliest possible position in the string.
and now, the rest of the principle squad 🫶 — these are the rules that decide which match you get once Principle 0 has already picked the earliest possible spot:
- Principle 1: in an alternation like
a|b|c..., the leftmost alternative that lets the whole regexp match is the one that wins. Leftmost first, periodt. - Principle 2: the maximal matching quantifiers
?,*,+, and{n,m}will in general match as much of the string as possible while still letting the whole regexp match. - Principle 3: if there are two or more elements in a regexp, the leftmost greedy quantifier (if any) will match as much of the string as possible while still allowing the whole regexp to match. Then the next leftmost greedy quantifier (if any) tries to match as much of what's left as possible while still allowing the whole regexp to match. And so on until every regexp element is satisfied.
So yeah, like we saw above, Principle 0 overrides all of these. The regexp matches as early as possible, and the other principles decide how it matches at that earliest character position. ✨
Here's these principles doing their thing in real life:
$x = "The programming republic of Perl";
$x =~ /^(.+)(e|r)(.*)$/; # matches,
# $1 = 'The programming republic of Pe'
# $2 = 'r'
# $3 = 'l'This regexp matches at the earliest string position, 'T'. You might think "e", being leftmost in the alternation, would be the one matched — but "r" gives the longest string in the first quantifier. slay for the greedy quantifier, honestly.
$x =~ /(m{1,2})(.*)$/; # matches,
# $1 = 'mm'
# $2 = 'ing republic of Perl'Here the earliest possible match is at the first 'm' in "programming". m{1,2} is the first quantifier, so it gets to match a maximal "mm".
$x =~ /.*(m{1,2})(.*)$/; # matches,
# $1 = 'm'
# $2 = 'ing republic of Perl'In this one the regexp matches at the start of the string. The first quantifier .* grabs as much as possible, leaving just a single 'm' for the second quantifier m{1,2}.
$x =~ /(.?)(m{1,2})(.*)$/; # matches,
# $1 = 'a'
# $2 = 'mm'
# $3 = 'ing republic of Perl'Here .? eats its maximal one character at the earliest possible position in the string — that's 'a' in "programming" — leaving m{1,2} the opportunity to match both "m"'s. And finally:
"aXXXb" =~ /(X*)/; # matches with $1 = ''because it can match zero copies of 'X' at the beginning of the string. If you definitely want at least one 'X', use X+, not X*. no notes 💅
Sometimes greed is not good, bestie. Sometimes we want quantifiers to match a minimal piece of string instead of a maximal piece. For this purpose, Larry Wall created the minimal match or non-greedy quantifiers ??, *?, +?, and {}?. These are the usual quantifiers with a ? appended to them. Their meanings are:
a??means: match'a'0 or 1 times. Try 0 first, then 1.a*?means: match'a'0 or more times, i.e., any number of times, but as few times as possible.a+?means: match'a'1 or more times, i.e., at least once, but as few times as possible.a{n,m}?means: match at leastntimes, not more thanmtimes, as few times as possible.a{n,}?means: match at leastntimes, but as few times as possible.a{n}?means: match exactlyntimes. Because we match exactlyntimes,a{n}?is equivalent toa{n}and is just there for notational consistency.
Let's look at the example above again, but with minimal quantifiers:
$x = "The programming republic of Perl";
$x =~ /^(.+?)(e|r)(.*)$/; # matches,
# $1 = 'Th'
# $2 = 'e'
# $3 = ' programming republic of Perl'The minimal string that will allow both the start-of-string anchor ^ and the alternation to match is "Th", with the alternation e|r matching "e". The second quantifier .* is free to gobble up the rest of the string. iconic behavior.
$x =~ /(m{1,2}?)(.*?)$/; # matches,
# $1 = 'm'
# $2 = 'ming republic of Perl'The first string position this regexp can match is at the first 'm' in "programming". At that position, the minimal m{1,2}? matches just one 'm'. Even though the second quantifier .*? would prefer to match no characters, it is constrained by the end-of-string anchor $ to match the rest of the string.
$x =~ /(.*?)(m{1,2}?)(.*)$/; # matches,
# $1 = 'The progra'
# $2 = 'm'
# $3 = 'ming republic of Perl'In this regexp you might expect the first minimal quantifier .*? to match the empty string, because it is not constrained by a ^ anchor to match the beginning of the word. But Principle 0 applies here, bestie. Because it is possible for the whole regexp to match at the start of the string, it will match at the start of the string. So the first quantifier has to match everything up to the first "m". The second minimal quantifier matches just one "m" and the third quantifier matches the rest of the string.
$x =~ /(.??)(m{1,2})(.*)$/; # matches,
# $1 = 'a'
# $2 = 'mm'
# $3 = 'ing republic of Perl'Just like in the previous regexp, the first quantifier .?? can match earliest at position 'a', so it does. The second quantifier is greedy, so it matches "mm", and the third matches the rest of the string.
We can modify Principle 3 above to take non-greedy quantifiers into account:
- Principle 3: if there are two or more elements in a regexp, the leftmost greedy (non-greedy) quantifier, if any, will match as much (little) of the string as possible while still allowing the whole regexp to match. The next leftmost greedy (non-greedy) quantifier, if any, will try to match as much (little) of the string remaining available to it as possible while still allowing the whole regexp to match. And so on until all the regexp elements are satisfied.
Just like alternation, quantifiers are also susceptible to backtracking. Here's a step-by-step analysis of this example:
$x = "the cat in the hat";
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 matches)- Start with the first letter in the string,
't'. - The first quantifier
.*starts out by matching the whole string'the cat in the hat'. 'a'in the regexp elementatdoesn't match the end of the string. Backtrack one character.'a'in the regexp elementatstill doesn't match the last letter of the string,'t', so backtrack one more character.- Now we can match the
'a'and the't'. - Move on to the third element
.*. Since we are at the end of the string and.*can match 0 times, assign it the empty string. - We are done!
Most of the time, all this moving forward and backtracking happens quickly and searching is fast. But there are some pathological regexps whose execution time exponentially grows with the size of the string 😭. A typical structure that blows up in your face is of the form:
/(a|b+)*/;The problem is the nested indeterminate quantifiers. There are many different ways of partitioning a string of length n between the + and *: one repetition with b+ of length n, two repetitions with the first b+ length k and the second with length n-k, m repetitions whose bits add up to length n, etc. In fact there are an exponential number of ways to partition a string as a function of its length. A regexp may get lucky and match early in the process, but if there is no match, Perl will try every possibility before giving up. So be careful with nested *'s, {n,m}'s, and +'s. The book Mastering Regular Expressions by Jeffrey Friedl gives a wonderful discussion of this and other efficiency issues.
bestie warning: nested*'s,{n,m}'s, and+'s can be a whole vibe killer for performance. you've been warned 💀
💅 Possessive quantifiers 💅
Backtracking during the relentless search for a match may be a waste of time, particularly when the match is bound to fail. Consider the simple pattern:
/^\w+\s+\w+$/; # a word, spaces, a wordWhenever this is applied to a string that doesn't quite meet the pattern's expectations — such as "abc " or "abc def " — the regex engine will backtrack, approximately once for each character in the string. But we know there is no way around taking all of the initial word characters to match the first repetition, that all spaces must be eaten by the middle part, and the same goes for the second word.
With the introduction of the possessive quantifiers in Perl 5.10, we have a way of instructing the regex engine not to backtrack, using the usual quantifiers with a + appended to them. This makes them greedy as well as stingy: once they succeed, they won't give anything back to permit another solution. They have the following meanings:
a{n,m}+means: match at leastntimes, not more thanmtimes, as many times as possible, and don't give anything up.a?+is short fora{0,1}+.a{n,}+means: match at leastntimes, but as many times as possible, and don't give anything up.a*+is short fora{0,}+anda++is short fora{1,}+.a{n}+means: match exactlyntimes. It is just there for notational consistency.
These possessive quantifiers represent a special case of a more general concept, the independent subexpression — see below.
As an example where a possessive quantifier is suitable, we consider matching a quoted string, as it appears in several programming languages. The backslash is used as an escape character that indicates the next character is to be taken literally, as another character for the string. Therefore, after the opening quote, we expect a (possibly empty) sequence of alternatives: either some character except an unescaped quote or backslash, or an escaped character.
/"(?:[^"\\]++|\\.)*+"/;🧚 Building a regexp 🧚
At this point we have all the basic regexp concepts covered, so let's give a more involved example of a regular expression. We will build a regexp that matches numbers.
The first task in building a regexp is to decide what we want to match and what we want to exclude. In our case, we want to match both integers and floating point numbers and we want to reject any string that isn't a number.
So the next task is to break the problem down into smaller problems that are easily converted into a regexp. That's the whole game, bestie — divide and conquer 🫶
The simplest case is integers. These consist of a sequence of digits, with an optional sign in front. The digits we can represent with \d+ and the sign can be matched with [+-]. Thus the integer regexp is
/[+-]?\d+/; # matches integersA floating point number potentially has a sign, an integral part, a decimal point, a fractional part, and an exponent. One or more of these parts is optional, so we need to check out the different possibilities. Floating point numbers which are in proper form include 123., 0.345, .34, -1e6, and 25.4E-72. As with integers, the sign out front is completely optional and can be matched by [+-]?. We can see that if there is no exponent, floating point numbers must have a decimal point, otherwise they are integers. We might be tempted to model these with \d*\.\d*, but this would also match just a single decimal point, which is not a number. So the three cases of floating point number without exponent are
/[+-]?\d+\./; # 1., 321., etc.
/[+-]?\.\d+/; # .1, .234, etc.
/[+-]?\d+\.\d+/; # 1.0, 30.56, etc.These can be combined into a single regexp with a three-way alternation:
/[+-]?(\d+\.\d+|\d+\.|\.\d+)/; # floating point, no exponentIn this alternation, it is important to put \d+\.\d+ before \d+\.. If \d+\. were first, the regexp would happily match that and ignore the fractional part of the number. order matters, bestie — the regexp is greedy AND lazy about being correct 😭
Now consider floating point numbers with exponents. The key observation here is that both integers and numbers with decimal points are allowed in front of an exponent. Then exponents, like the overall sign, are independent of whether we are matching numbers with or without decimal points, and can be decoupled from the mantissa. The overall form of the regexp now becomes clear:
/^(optional sign)(integer | f.p. mantissa)(optional exponent)$/;The exponent is an e or E, followed by an integer. So the exponent regexp is
/[eE][+-]?\d+/; # exponentPutting all the parts together, we get a regexp that matches numbers:
/^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/; # Ta da!Long regexps like this may impress your friends, but can be hard to decipher. it's giving unreadable group chat screenshot at 3am ngl In complex situations like this, the //x modifier for a match is invaluable. It allows one to put nearly arbitrary whitespace and comments into a regexp without affecting their meaning. Using it, we can rewrite our 'extended' regexp in the more pleasing form
/^
[+-]? # first, match an optional sign
( # then match integers or f.p. mantissas:
\d+\.\d+ # mantissa of the form a.b
|\d+\. # mantissa of the form a.
|\.\d+ # mantissa of the form .b
|\d+ # integer of the form a
)
([eE][+-]?\d+)? # finally, optionally match an exponent
$/x;If whitespace is mostly irrelevant, how does one include space characters in an extended regexp? The answer is to backslash it \ or put it in a character class [ ]. The same thing goes for pound signs: use \# or [#]. For instance, Perl allows a space between the sign and the mantissa or integer, and we could add this to our regexp as follows:
/^
[+-]?\ * # first, match an optional sign *and space*
( # then match integers or f.p. mantissas:
\d+\.\d+ # mantissa of the form a.b
|\d+\. # mantissa of the form a.
|\.\d+ # mantissa of the form .b
|\d+ # integer of the form a
)
([eE][+-]?\d+)? # finally, optionally match an exponent
$/x;In this form, it is easier to see a way to simplify the alternation. Alternatives 1, 2, and 4 all start with \d+, so it could be factored out:
/^
[+-]?\ * # first, match an optional sign
( # then match integers or f.p. mantissas:
\d+ # start out with a ...
(
\.\d* # mantissa of the form a.b or a.
)? # ? takes care of integers of the form a
|\.\d+ # mantissa of the form .b
)
([eE][+-]?\d+)? # finally, optionally match an exponent
$/x;or written in the compact form,
/^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/;This is our final regexp. To recap, we built a regexp by
- specifying the task in detail,
- breaking down the problem into smaller parts,
- translating the small parts into regexps,
- combining the regexps,
- and optimizing the final combined regexp.
These are also the typical steps involved in writing a computer program. This makes perfect sense, because regular expressions are essentially programs written in a little computer language that specifies patterns. regexps ARE programs, periodt 👑
✨ Using regular expressions in Perl ✨
The last topic of Part 1 briefly covers how regexps are used in Perl programs. Where do they fit into Perl syntax? 🦋
We have already introduced the matching operator in its default /regexp/ and arbitrary delimiter m!regexp! forms. We have used the binding operator =~ and its negation !~ to test for string matches. Associated with the matching operator, we have discussed the single line //s, multi-line //m, case-insensitive //i and extended //x modifiers. There are a few more things you might want to know about matching operators.
💅 Prohibiting substitution
If you change $pattern after the first substitution happens, Perl will ignore it. If you don't want any substitutions at all, use the special delimiter m'':
@pattern = ('Seuss');
while (<>) {
print if m'@pattern'; # matches literal '@pattern', not 'Seuss'
}Similar to strings, m'' acts like apostrophes on a regexp; all other m delimiters act like quotes. If the regexp evaluates to the empty string, the regexp in the last successful match is used instead. So we have
"dog" =~ /d/; # 'd' matches
"dogbert =~ //; # this matches the 'd' regexp used beforeheads up bestie — that empty-regexp thing is a real gotcha. An empty pattern doesn't match "nothing", it reuses whatever matched last. sneaky icon behavior 💀
🎀 Global matching
The final two modifiers we will discuss here, //g and //c, concern multiple matches. The modifier //g stands for global matching and allows the matching operator to match within a string as many times as possible. In scalar context, successive invocations against a string will have //g jump from match to match, keeping track of position in the string as it goes along. You can get or set the position with the pos() function. ⭐
The use of //g is shown in the following example. Suppose we have a string that consists of words separated by spaces. If we know how many words there are in advance, we could extract the words using groupings:
$x = "cat dog house"; # 3 words
$x =~ /^\s*(\w+)\s+(\w+)\s+(\w+)\s*$/; # matches,
# $1 = 'cat'
# $2 = 'dog'
# $3 = 'house'But what if we had an indeterminate number of words? This is the sort of task //g was made for. To extract all words, form the simple regexp (\w+) and loop over all matches with /(\w+)/g:
while ($x =~ /(\w+)/g) {
print "Word is $1, ends at position ", pos $x, "\n";
}prints
Word is cat, ends at position 3
Word is dog, ends at position 7
Word is house, ends at position 13A failed match or changing the target string resets the position. If you don't want the position reset after failure to match, add the //c, as in /regexp/gc. The current position in the string is associated with the string, not the regexp. This means that different strings have different positions and their respective positions can be set or read independently. the position lives on the STRING, not the pattern — that's the whole tea ☕
In list context, //g returns a list of matched groupings, or if there are no groupings, a list of matches to the whole regexp. So if we wanted just the words, we could use
@words = ($x =~ /(\w+)/g); # matches,
# $word[0] = 'cat'
# $word[1] = 'dog'
# $word[2] = 'house'Closely associated with the //g modifier is the \G anchor. The \G anchor matches at the point where the previous //g match left off. \G allows us to easily do context-sensitive matching:
$metric = 1; # use metric units
...
$x = <FILE>; # read in measurement
$x =~ /^([+-]?\d+)\s*/g; # get magnitude
$weight = $1;
if ($metric) { # error checking
print "Units error!" unless $x =~ /\Gkg\./g;
}
else {
print "Units error!" unless $x =~ /\Glbs\./g;
}
$x =~ /\G\s+(widget|sprocket)/g; # continue processingThe combination of //g and \G allows us to process the string a bit at a time and use arbitrary Perl logic to decide what to do next. Currently, the \G anchor is only fully supported when used to anchor to the start of the pattern. only-at-the-start, bestie — don't go sticking \G in the middle and expecting main character energy 🙈
\G is also invaluable in processing fixed-length records with regexps. Suppose we have a snippet of coding region DNA, encoded as base pair letters ATCGTTGAAT... and we want to find all the stop codons TGA. In a coding region, codons are 3-letter sequences, so we can think of the DNA snippet as a sequence of 3-letter records. The naive regexp
# expanded, this is "ATC GTT GAA TGC AAA TGA CAT GAC"
$dna = "ATCGTTGAATGCAAATGACATGAC";
$dna =~ /TGA/;doesn't work; it may match a TGA, but there is no guarantee that the match is aligned with codon boundaries, e.g., the substring GTT GAA gives a match. A better solution is
while ($dna =~ /(\w\w\w)*?TGA/g) { # note the minimal *?
print "Got a TGA stop codon at position ", pos $dna, "\n";
}which prints
Got a TGA stop codon at position 18
Got a TGA stop codon at position 23Position 18 is good, but position 23 is bogus. What happened? 😭
The answer is that our regexp works well until we get past the last real match. Then the regexp will fail to match a synchronized TGA and start stepping ahead one character position at a time, not what we want. The solution is to use \G to anchor the match to the codon alignment:
while ($dna =~ /\G(\w\w\w)*?TGA/g) {
print "Got a TGA stop codon at position ", pos $dna, "\n";
}This prints
Got a TGA stop codon at position 18which is the correct answer. This example illustrates that it is important not only to match what is desired, but to reject what is not desired. matching the right thing is half the battle — rejecting the wrong thing is the other half, and she ATE 🔥
Ngl bestie, there are a few other regexp modifiers floating around out there — like //o, //d, and //l — but their whole specialized thing is honestly beyond the scope of this intro. We're moving on! 💅
🔍 Search and replace 🔍
Okay so regexps also slay at search-and-replace action in Perl — it's literally their main character moment. The operator for this is s///, and the general form is s/regexp/replacement/modifiers, with all the regexp knowledge and modifiers we already talked about applying here too. The replacement is a Perl double-quoted string that swaps in for whatever the regexp matched. The =~ operator shows up here as well, doing its job of associating a string with s///. And if you're matching against $_, you can just drop the $_ =~ entirely. 💖
If there's a match, s/// returns the number of substitutions it made; otherwise it returns false. Here are a few examples to vibe with:
$x = "Time to feed the cat!";
$x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
if ($x =~ s/^(Time.*hacker)!$/$1 now!/) {
$more_insistent = 1;
}
$y = "'quoted words'";
$y =~ s/^'(.*)'$/$1/; # strip single quotes,
# $y contains "quoted words"In that last example the whole string got matched, but only the part inside the single quotes was grouped. With the s/// operator, the matched variables $1, $2, etc. are immediately available to use in the replacement expression — so we use $1 to replace the quoted string with just the thing that was quoted. Iconic. ✨
And with the global modifier, s///g will search and replace all occurrences of the regexp in the string:
$x = "I batted 4 for 4";
$x =~ s/4/four/; # doesn't do it all:
# $x contains "I batted four for 4"
$x = "I batted 4 for 4";
$x =~ s/4/four/g; # does it all:
# $x contains "I batted four for four"If you're more of a 'regex' girlie than a 'regexp' girlie, you could use this little program to swap it out:
% cat > simple_replace
#!/usr/bin/perl
$regexp = shift;
$replacement = shift;
while (<>) {
s/$regexp/$replacement/g;
print;
}
^D
% simple_replace regexp regex perlretut.podIn simple_replace we used the s///g modifier to replace all occurrences of the regexp on each line. (Even though the regular expression appears inside a loop, Perl is smart enough to compile it only once — no cap, it's efficient like that.) Just like in simple_grep, both the print and the s/$regexp/$replacement/g are using $_ implicitly. 🫶
If you don't want s/// to change your original variable, you can use the non-destructive substitute modifier, s///r. This switches up the behavior so that s///r returns the final substituted string (instead of the number of substitutions):
$x = "I like dogs.";
$y = $x =~ s/dogs/cats/r;
print "$x $y\n";That example prints I like dogs. I like cats. Notice that the original $x variable was not affected, bestie. The overall result of the substitution is stored in $y instead. And if the substitution doesn't affect anything, the original string just comes right back to you:
$x = "I like dogs.";
$y = $x =~ s/elephants/cougars/r;
print "$x $y\n"; # prints "I like dogs. I like dogs."One other interesting thing the s///r flag unlocks is chaining substitutions — which is lowkey so satisfying:
$x = "Cats are great.";
print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
# prints "Hedgehogs are great."Another modifier that's specifically for search and replace is the s///e evaluation modifier. s///e treats the replacement text as Perl code, not as a double-quoted string. Whatever value the code returns gets substituted in for the matched substring. s///e is here for when you need to do a little computation mid-replacement. This example counts character frequencies in a line:
$x = "Bill the cat";
$x =~ s/(.)/$chars{$1}++;$1/eg; # final $1 replaces char with itself
print "frequency of '$_' is $chars{$_}\n"
foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);This prints:
frequency of ' ' is 2
frequency of 't' is 2
frequency of 'l' is 2
frequency of 'B' is 1
frequency of 'c' is 1
frequency of 'e' is 1
frequency of 'h' is 1
frequency of 'i' is 1
frequency of 'a' is 1Just like with the match m// operator, s/// can use other delimiters too — such as s!!! and s{}{}, and even s{}//. And if single quotes are used, like s''', then the regexp and replacement are treated as single-quoted strings with no variable substitutions happening. s/// in list context returns the same thing as in scalar context — i.e., the number of matches. 🎀
✂️ The split function ✂️
The split() function is another spot where a regexp comes out to play. split /regexp/, string, limit separates the string operand into a list of substrings and returns that list. The regexp has to be designed to match whatever makes up the separators for the substrings you want. The limit, if you include it, constrains the split into no more than limit number of strings. For example, to split a string into words, you do:
$x = "Calvin and Hobbes";
@words = split /\s+/, $x; # $word[0] = 'Calvin'
# $word[1] = 'and'
# $word[2] = 'Hobbes'If you use the empty regexp //, the regexp always matches and your string gets split into individual characters. And if the regexp has groupings, then the resulting list contains the matched substrings from those groupings as well. For instance:
$x = "/usr/bin/perl";
@dirs = split m!/!, $x; # $dirs[0] = ''
# $dirs[1] = 'usr'
# $dirs[2] = 'bin'
# $dirs[3] = 'perl'
@parts = split m!(/)!, $x; # $parts[0] = ''
# $parts[1] = '/'
# $parts[2] = 'usr'
# $parts[3] = '/'
# $parts[4] = 'bin'
# $parts[5] = '/'
# $parts[6] = 'perl'Since the first character of $x matched the regexp, split prepended an empty initial element to the list. Sneaky but correct. 💅
If you've read this far — congratulations, bestie! 🎉 You now have all the basic tools you need to use regular expressions to solve a whole wide range of text processing problems. If this is your first time through the tutorial, why not stop right here and go play around with regexps for a while.... Part 2 is all about the more esoteric aspects of regular expressions, and those concepts are genuinely not needed right at the start. Enjoy the vibes first.
⚡ Part 2: Power tools ⚡
OK so you know the basics of regexps and you want more. If matching regular expressions is like a walk in the woods, then the tools from Part 1 are like topo maps and a compass — the basic stuff we use all the time. Most of the tools in Part 2 are more like flare guns and satellite phones. You don't use them too often on a hike, but when you're stuck? They can be absolutely invaluable. 🧚
What follows are the more advanced, less used, or sometimes straight-up esoteric capabilities of Perl regexps. In Part 2 we're gonna assume you're comfy with the basics and zero in on the advanced features. Let's go. 🔥
🔤 More on characters, strings, and character classes 🔤
There are a bunch of escape sequences and character classes that we haven't covered yet — time to fix that.
There are several escape sequences that convert characters or strings between upper and lower case, and they're available inside patterns too. \l and \u convert the next character to lower or upper case, respectively:
$x = "perl";
$string =~ /\u$x/; # matches 'Perl' in $string
$x = "M(rs?|s)\\."; # note the double backslash
$string =~ /\l$x/; # matches 'mr.', 'mrs.', and 'ms.',A \L or \U means a lasting conversion of case, until it's terminated by \E or overthrown by another \U or \L:
$x = "This word is in lower case:\L SHOUT\E";
$x =~ /shout/; # matches
$x = "I STILL KEYPUNCH CARDS FOR MY 360"
$x =~ /\Ukeypunch/; # matches punch card stringIf there's no \E, case conversion just keeps going until the end of the string. The regexps \L\u$word or \u\L$word convert the first character of $word to uppercase and the rest of the characters to lowercase. Cute, right? 🎀
Control characters can be escaped with \c, so a control-Z character would be matched with \cZ. The escape sequence \Q...\E quotes, or protects, most non-alphabetic characters. For instance:
$x = "\QThat !^*&%~& cat!";
$x =~ /\Q!^*&%~&\E/; # check for rough languageIt does not protect $ or @, though — so variables can still be substituted. Good to know. 💖
Heads up bestie:\Q,\L,\l,\U,\uand\Eare actually part of double-quotish syntax, and not part of regexp syntax proper. They'll work if they appear in a regular expression embedded directly in a program, but not when they're contained in a string that gets interpolated into a pattern.
With the advent of 5.6.0, Perl regexps can handle way more than just the standard ASCII character set. Perl now supports Unicode — a standard for representing the alphabets from virtually all of the world's written languages, plus a whole host of symbols. Perl's text strings are Unicode strings, so they can contain characters with a value (codepoint or character number) higher than 255. ✨
So what does this mean for regexps? Well, regexp users don't need to know much about Perl's internal representation of strings. But they do need to know 1) how to represent Unicode characters in a regexp, and 2) that a matching operation will treat the string being searched as a sequence of characters, not bytes. The answer to 1) is that Unicode characters greater than chr(255) are represented using the \x{hex} notation, because \x hex (without curly braces) doesn't go further than 255. (Starting in Perl 5.14, if you're an octal fan, you can also use \o{oct}.) 🦋
/\x{263a}/; # match a Unicode smiley face :)NOTE: In Perl 5.6.0 it used to be that you needed to sayuse utf8to use any Unicode features. This is no longer the case — for almost all Unicode processing, the explicitutf8pragma is not needed. (The only case where it matters is if your Perl script is in Unicode and encoded in UTF-8, then an explicituse utf8is needed.)
Slay, you're officially Unicode-literate now. 💜
figuring out the hex code for a Unicode char by hand (or decoding someone else's hex Unicode regexp 😭) is genuinely about as fun as writing machine code. so there's a better way: the named character escape \N{name}, where name is the character's name per the Unicode standard. like, say we wanted to represent or match the astrological sign for the planet Mercury — we'd just:
use charnames ":full"; # use named chars with Unicode full names
$x = "abc\N{MERCURY}def";
$x =~ /\N{MERCURY}/; # matchesyou can also use short names, or restrict names to a certain alphabet:
use charnames ':full';
print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n";
use charnames ":short";
print "\N{greek:Sigma} is an upper-case sigma.\n";
use charnames qw(greek);
print "\N{sigma} is Greek sigma\n";the full list of full names lives in NamesList.txt in the Unicode standard, available at <http://www.unicode.org/Public/UNIDATA/>.
so the answer to requirement 2), as of 5.6.0, is that a regexp (mostly) uses Unicode characters. (messy backward-compat reasons mean most but not all semantics of a match assume Unicode — unless, starting in Perl 5.14, you tell it to go full Unicode. you can do that explicitly with the //u modifier, or ask Perl to apply it implicitly to all regexes in a scope via use 5.012 (or higher) or use feature 'unicode_strings'.) if you wanna handle Unicode properly, make sure one of those is true, mmkay? internally it gets encoded to bytes using either UTF-8 or a native 8-bit encoding depending on the string's history, but conceptually it's a sequence of characters, not bytes. see perlunitut for a whole tutorial on that.
now bestie, let's talk Unicode character classes. just like with chars, there are named Unicode character classes represented by the \p{name} escape. its bestie is \P{name}, which is the negation of \p{name}. for example, matching lower and uppercase:
use charnames ":full"; # use named chars with Unicode full names
$x = "BOB";
$x =~ /^\p{IsUpper}/; # matches, uppercase char class
$x =~ /^\P{IsUpper}/; # doesn't match, char class sans uppercase
$x =~ /^\p{IsLower}/; # doesn't match, lowercase char class
$x =~ /^\P{IsLower}/; # matches, char class sans lowercase(the Is is optional, iykyk.)
here's the mapping between some Perl named classes and the traditional Unicode classes:
Perl class name Unicode class name or regular expression
IsAlpha /^[LM]/
IsAlnum /^[LMN]/
IsASCII $code <= 127
IsCntrl /^C/
IsBlank $code =~ /^(0020|0009)$/ || /^Z[^lp]/
IsDigit Nd
IsGraph /^([LMNPS]|Co)/
IsLower Ll
IsPrint /^([LMNPS]|Co|Zs)/
IsPunct /^P/
IsSpace /^Z/ || ($code =~ /^(0009|000A|000B|000C|000D)$/
IsSpacePerl /^Z/ || ($code =~ /^(0009|000A|000C|000D|0085|2028|2029)$/
IsUpper /^L[ut]/
IsWord /^[LMN]/ || $code eq "005F"
IsXDigit $code =~ /^00(3[0-9]|[46][1-6])$/you can also use the official Unicode class names with \p and \P — like \p{L} for Unicode 'letters', \p{Lu} for uppercase letters, or \P{Nd} for non-digits. and if a name is just one letter, you can drop the braces entirely: \pM is the character class of Unicode 'marks' (accent marks etc.). full list is in perlunicode. 💜
Unicode also gets separated into various sets of characters you can test with \p{...} (in) and \P{...} (not in). to test whether a char is (or isn't) part of a script, you use the script name — \p{Latin}, \p{Greek}, \P{Katakana}, iconic behaviour honestly.
everything we just described is the single form of \p{...} classes. there's also a compound form you might run into: they look like \p{name=value} or \p{name:value} (equals sign and colon are interchangeable, slay). these are more general than the single form, and honestly most of the single forms are just Perl-defined shortcuts for common compound forms. for example the script examples above could be written equivalently as \p{Script=Latin}, \p{Script:Greek}, and \P{script=katakana} (case is irrelevant between the {} braces). you may never have to use compound forms, but sometimes it's necessary, and using them can make your code easier to understand. 🫶
\X is an abbreviation for a character class that's a Unicode extended grapheme cluster. it represents a "logical character": something that looks like one single character but may be represented internally by more than one. example: using Unicode full names, A + COMBINING RING is a grapheme cluster with base character A and combining character COMBINING RING — which translates in Danish to A with the little circle on top, like in the word Angstrom. 🎀
for the full and latest info about Unicode, see the latest Unicode standard, or the Unicode Consortium's website <http://www.unicode.org>.
as if all those classes weren't enough, Perl also defines POSIX-style character classes. these have the form [:name:], where name is the POSIX class name. the POSIX classes are alpha, alnum, ascii, cntrl, digit, graph, lower, print, punct, space, upper, and xdigit, plus two extensions: word (a Perl extension matching \w) and blank (a GNU extension). the //a modifier restricts these to matching only in the ASCII range; otherwise they can match the same as their corresponding Perl Unicode classes: [:upper:] is the same as \p{IsUpper}, etc. (there are some exceptions and gotchas here — see perlrecharclass for the full discussion.) [:digit:], [:word:], and [:space:] correspond to the familiar \d, \w, and \s character classes. to negate a POSIX class, put a ^ in front of the name, so e.g. [:^digit:] corresponds to \D and, under Unicode, \P{IsDigit}. Unicode and POSIX character classes can be used just like \d, except that POSIX character classes can only be used inside of a character class:
/\s+[abc[:digit:]xyz]\s*/; # match a,b,c,x,y,z, or a digit
/^=item\s[[:digit:]]/; # match '=item',
# followed by a space and a digit
/\s+[abc\p{IsDigit}xyz]\s+/; # match a,b,c,x,y,z, or a digit
/^=item\s\p{IsDigit}/; # match '=item',
# followed by a space and a digitwhew! that is all the rest of the characters and character classes. 😮💨
🎀 Compiling and saving regular expressions 🎀
back in Part 1 we mentioned that Perl compiles a regexp into a compact sequence of opcodes. so a compiled regexp is a data structure you can store once and reuse again and again. the regexp quote qr// does exactly that: qr/string/ compiles the string as a regexp and transforms the result into a form you can assign to a variable:
$reg = qr/foo+bar?/; # reg contains a compiled regexpthen $reg can be used as a regexp:
$x = "fooooba";
$x =~ $reg; # matches, just like /foo+bar?/
$x =~ /$reg/; # same thing, alternate formand $reg can also be interpolated into a bigger regexp:
$x =~ /(abc)?$reg/; # still matchesjust like with the matching operator, the regexp quote can use different delimiters, e.g. qr!!, qr{} or qr~~. apostrophes as delimiters (qr'') inhibit any interpolation — noted. 🙈
pre-compiled regexps are so useful for creating dynamic matches that don't need to be recompiled every time they're hit. using them, here's a grep_step program which greps for a sequence of patterns, advancing to the next pattern as soon as one has been satisfied:
% cat > grep_step
#!/usr/bin/perl
# grep_step - match <number> regexps, one after the other
# usage: multi_grep <number> regexp1 regexp2 ... file1 file2 ...
$number = shift;
$regexp[$_] = shift foreach (0..$number-1);
@compiled = map qr/$_/, @regexp;
while ($line = <>) {
if ($line =~ /$compiled[0]/) {
print $line;
shift @compiled;
last unless @compiled;
}
}
^D
% grep_step 3 shift print last grep_step
$number = shift;
print $line;
last unless @compiled;storing pre-compiled regexps in an array @compiled lets us just loop through them with zero recompilation — flexibility without sacrificing speed. periodt. ⭐
🌈 Composing regular expressions at runtime 🌈
backtracking is more efficient than doing repeated tries with different regular expressions. so if you have several regexps and a match with any of them is acceptable, you can combine them into a set of alternatives. and if the individual expressions are input data, you can do that by programming a join operation. let's exploit that idea in an improved version of the simple_grep program — one that matches multiple patterns:
% cat > multi_grep
#!/usr/bin/perl
# multi_grep - match any of <number> regexps
# usage: multi_grep <number> regexp1 regexp2 ... file1 file2 ...
$number = shift;
$regexp[$_] = shift foreach (0..$number-1);
$pattern = join '|', @regexp;
while ($line = <>) {
print $line if $line =~ /$pattern/;
}
^D
% multi_grep 2 shift for multi_grep
$number = shift;
$regexp[$_] = shift foreach (0..$number-1);sometimes it's actually advantageous to construct a pattern from the input that's to be analyzed and use the permissible values on the left-hand side of the matching operations. as an example of this somewhat paradoxical situation, let's assume our input contains a command verb which should match one out of a set of available command verbs — with the extra twist that commands may be abbreviated as long as the given string is unique. the program below demonstrates the basic algorithm. 🦋
% cat > keymatch
#!/usr/bin/perl
$kwds = 'copy compare list print';
while( $command = <> ){
$command =~ s/^\s+|\s+$//g; # trim leading and trailing spaces
if( ( @matches = $kwds =~ /\b$command\w*/g ) == 1 ){
print "command: '@matches'\n";
} elsif( @matches == 0 ){
print "no such command: '$command'\n";
} else {
print "not unique: '$command' (could be one of: @matches)\n";
}
}
^D
% keymatch
li
command: 'list'
co
not unique: 'co' (could be one of: copy compare)
printer
no such command: 'printer'So bestie, notice the flip 💅 — instead of trying to match the input against the keywords, we match the combined set of keywords against the input. Iconic, honestly. The pattern matching operation "$kwds =~ /\b($command\w*)/g" does several things at the same time:
- It makes sure that the given command begins where a keyword begins (
\b). - It tolerates abbreviations due to the added
\w*. - It tells us the number of matches (
scalar @matches) and all the keywords that were actually matched.
You could hardly ask for more. Like, genuinely. 🌟
🌸 Embedding comments and modifiers in a regular expression
Starting with this section, we will be discussing Perl's set of extended patterns. These are extensions to the traditional regular expression syntax that provide powerful new tools for pattern matching. We have already seen extensions in the form of the minimal matching constructs ??, *?, +?, {n,m}?, and {n,}?. Most of the extensions below have the form (?char...), where the char is a character that determines the type of extension. iykyk 🧚
The first extension is an embedded comment (?#text). This embeds a comment into the regular expression without affecting its meaning. The comment should not have any closing parentheses in the text — so like, no stray ) in your little love note, okay? An example is:
/(?# Match an integer:)[+-]?\d+/;This style of commenting has been largely superseded by the raw, freeform commenting that is allowed with the //x modifier. (RIP to the old way, it ate, but we moved on 💀)
Most modifiers, such as //i, //m, //s and //x (or any combination thereof) can also be embedded in a regexp using (?i), (?m), (?s), and (?x). For instance:
/(?i)yes/; # match 'yes' case insensitively
/yes/i; # same thing
/(?x)( # freeform version of an integer regexp
[+-]? # match an optional sign
\d+ # match a sequence of digits
)
/x;Embedded modifiers can have two important advantages over the usual modifiers. 👑
- Embedded modifiers allow a custom set of modifiers to each regexp pattern. This is great for matching an array of regexps that must have different modifiers:
$pattern[0] = '(?i)doctor';
$pattern[1] = 'Johnson';
...
while (<>) {
foreach $patt (@pattern) {
print if /$patt/;
}
}- The second advantage is that embedded modifiers (except
//p, which modifies the entire regexp) only affect the regexp inside the group the embedded modifier is contained in. So grouping can be used to localize the modifier's effects:
/Answer: ((?i)yes)/; # matches 'Answer: yes', 'Answer: YES', etc.Embedded modifiers can also turn off any modifiers already present by using, e.g., (?-i). Modifiers can also be combined into a single expression, e.g., (?s-i) turns on single line mode and turns off case insensitivity. Queen of multitasking 💅
Embedded modifiers may also be added to a non-capturing grouping. (?i-m:regexp) is a non-capturing grouping that matches regexp case insensitively and turns off multi-line mode. Obsessed with her tbh.
🦋 Looking ahead and looking behind
This section concerns the lookahead and lookbehind assertions. First, a little background. 📖
In Perl regular expressions, most regexp elements 'eat up' a certain amount of string when they match. For instance, the regexp element [abc}] eats up one character of the string when it matches, in the sense that Perl moves to the next character position in the string after the match. There are some elements, however, that don't eat up characters (advance the character position) if they match. The examples we have seen so far are the anchors. The anchor ^ matches the beginning of the line, but doesn't eat any characters. Similarly, the word boundary anchor \b matches wherever a character matching \w is next to a character that doesn't, but it doesn't eat up any characters itself.
Anchors are examples of zero-width assertions: zero-width, because they consume no characters, and assertions, because they test some property of the string. In the context of our walk in the woods analogy to regexp matching, most regexp elements move us along a trail, but anchors have us stop a moment and check our surroundings. If the local environment checks out, we can proceed forward. But if the local environment doesn't satisfy us, we must backtrack. 🚶♀️🌲
Checking the environment entails either looking ahead on the trail, looking behind, or both:
^looks behind, to see that there are no characters before.$looks ahead, to see that there are no characters after.\blooks both ahead and behind, to see if the characters on either side differ in their "word-ness".
The lookahead and lookbehind assertions are generalizations of the anchor concept. Lookahead and lookbehind are zero-width assertions that let us specify which characters we want to test for. The lookahead assertion is denoted by (?=regexp) and the lookbehind assertion is denoted by (?<=fixed-regexp). Some examples are:
$x = "I catch the housecat 'Tom-cat' with catnip";
$x =~ /cat(?=\s)/; # matches 'cat' in 'housecat'
@catwords = ($x =~ /(?<=\s)cat\w+/g); # matches,
# $catwords[0] = 'catch'
# $catwords[1] = 'catnip'
$x =~ /\bcat\b/; # matches 'cat' in 'Tom-cat'
$x =~ /(?<=\s)cat(?=\s)/; # doesn't match; no isolated 'cat' in
# middle of $xNote that the parentheses in (?=regexp) and (?<=regexp) are non-capturing, since these are zero-width assertions. Thus in the second regexp, the substrings captured are those of the whole regexp itself. 🫶
Lookahead (?=regexp) can match arbitrary regexps, but lookbehind (?<=fixed-regexp) only works for regexps of fixed width, i.e., a fixed number of characters long. Thus (?<=(ab|bc)) is fine, but (?<=(ab)*) is not. Don't come for me, that's just the rules. 😭
The negated versions of the lookahead and lookbehind assertions are denoted by (?!regexp) and (?<!fixed-regexp) respectively. They evaluate true if the regexps do not match:
$x = "foobar";
$x =~ /foo(?!bar)/; # doesn't match, 'bar' follows 'foo'
$x =~ /foo(?!baz)/; # matches, 'baz' doesn't follow 'foo'
$x =~ /(?<!\s)foo/; # matches, there is no \s before 'foo'Heads up bestie 💀 — the\Cis unsupported in lookbehind, because the already treacherous definition of\Cwould become even more so when going backwards. Like, it's giving danger, we're not doing that.
Here is an example where a string containing blank-separated words, numbers and single dashes is to be split into its components. Using /\s+/ alone won't work, because spaces are not required between dashes, or a word or a dash. Additional places for a split are established by looking ahead and behind:
$str = "one two - --6-8";
@toks = split / \s+ # a run of spaces
| (?<=\S) (?=-) # any non-space followed by '-'
| (?<=-) (?=\S) # a '-' followed by any non-space
/x, $str; # @toks = qw(one two - - - 6 - 8)🔥 Using independent subexpressions to prevent backtracking
Independent subexpressions are regular expressions, in the context of a larger regular expression, that function independently of the larger regular expression. That is, they consume as much or as little of the string as they wish without regard for the ability of the larger regexp to match. Independent subexpressions are represented by (?>regexp). Main character energy, honestly — they do not care about the group chat. 💅
We can illustrate their behavior by first considering an ordinary regexp:
$x = "ab";
$x =~ /a*ab/; # matchesThis obviously matches, but in the process of matching, the subexpression a* first grabbed the a. Doing so, however, wouldn't allow the whole regexp to match, so after backtracking, a* eventually gave back the a and matched the empty string. Here, what a* matched was dependent on what the rest of the regexp matched.
Contrast that with an independent subexpression:
$x =~ /(?>a*)ab/; # doesn't match!The independent subexpression (?>a*) doesn't care about the rest of the regexp, so it sees an a and grabs it. Then the rest of the regexp ab cannot match. Because (?>a*) is independent, there is no backtracking and the independent subexpression does not give up its a. Thus the match of the regexp as a whole fails. 😤
A similar behavior occurs with completely independent regexps:
$x = "ab";
$x =~ /a*/g; # matches, eats an 'a'
$x =~ /\Gab/g; # doesn't match, no 'a' availableHere //g and \G create a 'tag team' handoff of the string from one regexp to the other. Regexps with an independent subexpression are much like this, with a handoff of the string to the independent subexpression, and a handoff of the string back to the enclosing regexp. 🤝
The ability of an independent subexpression to prevent backtracking can be quite useful. Suppose we want to match a non-empty string enclosed in parentheses up to two levels deep. Then the following regexp matches:
$x = "abc(de(fg)h"; # unbalanced parentheses
$x =~ /\( ( [^()]+ | \([^()]*\) )+ \)/x;The regexp matches an open parenthesis, one or more copies of an alternation, and a close parenthesis. The alternation is two-way, with the first alternative [^()]+ matching a substring with no parentheses and the second alternative \([^()]*\) matching a substring delimited by parentheses.
The problem with this regexp is that it is pathological: it has nested indeterminate quantifiers of the form (a+|b)+. We discussed in Part 1 how nested quantifiers like this could take an exponentially long time to execute if there was no match possible. To prevent the exponential blowup, we need to prevent useless backtracking at some point. This can be done by enclosing the inner quantifier as an independent subexpression:
$x =~ /\( ( (?>[^()]+) | \([^()]*\) )+ \)/x;Here, (?>[^()]+) breaks the degeneracy of string partitioning by gobbling up as much of the string as possible and keeping it. Then match failures fail much more quickly. Slay. ⚡
🎀 Conditional expressions 🎀
okay so a conditional expression is basically an if-then-else situation for regexes — it lets you pick which patterns get matched based on some condition. There are two flavors, bestie:
(?(condition)yes-regexp)(?(condition)yes-regexp|no-regexp)
The first one is giving if () {} energy in Perl. If the condition is true, the yes-regexp gets matched. If it's false, yes-regexp gets skipped and Perl moves on to the next regexp element. The second form is the if () {} else {} version: if condition is true, yes-regexp matches; otherwise, no-regexp matches. 💅
The condition can show up in several forms. First form: just an integer in parentheses, (integer). It's true if the corresponding backreference \integer matched earlier in the regexp. Same deal works with a name tied to a capture group, written as (<name>) or ('name'). Second form: a bare zero-width assertion (?...), either a lookahead, a lookbehind, or a code assertion (we get into that in the next section). Third set of forms gives you tests that return true if the expression is executed inside a recursion (R), or is being called from some capturing group referenced either by number (R1), (R2),... or by name (R&name). ✨
The integer or name form of condition lets us choose with more flexibility what to match based on what matched earlier in the regexp. This searches for words of the form $x$x or $x$y$y$x:
% simple_grep '^(\w+)(\w+)?(?(2)\g2\g1|\g1)$' /usr/dict/words
beriberi
coco
couscous
deed
...
toot
toto
tutuThe lookbehind condition lets an earlier part of the match influence a later part of the match, along with backreferences. For instance:
/[ATGC]+(?(?<=AA)G|C)$/;matches a DNA sequence such that it either ends in AAG, or some other base pair combination and C. Note that the form is (?(?<=AA)G|C) and not (?((?<=AA))G|C); for the lookahead, lookbehind or code assertions, the parentheses around the conditional are not needed. 🧬
🧚 Defining named patterns 🧚
Some regular expressions use identical subpatterns in several places. Starting with Perl 5.10, it is possible to define named subpatterns in a section of the pattern so that they can be called up by name anywhere in the pattern. This syntactic pattern for this definition group is (?(DEFINE)(?<name>pattern)...). An insertion of a named pattern is written as (?&name). 🎀
The example below illustrates this feature using the pattern for floating point numbers that was presented earlier on. The three subpatterns that are used more than once are the optional sign, the digit sequence for an integer and the decimal fraction. The DEFINE group at the end of the pattern contains their definition. Notice that the decimal fraction pattern is the first place where we can reuse the integer pattern.
/^ (?&osg)\ * ( (?&int)(?&dec)? | (?&dec) )
(?: [eE](?&osg)(?&int) )?
$
(?(DEFINE)
(?<osg>[-+]?) # optional sign
(?<int>\d++) # integer
(?<dec>\.(?&int)) # decimal fraction
)/x🔁 Recursive patterns 🔁
This feature (introduced in Perl 5.10) significantly extends the power of Perl's pattern matching. By referring to some other capture group anywhere in the pattern with the construct (?group-ref), the pattern within the referenced group is used as an independent subpattern in place of the group reference itself. Because the group reference may be contained within the group it refers to, it is now possible to apply pattern matching to tasks that hitherto required a recursive parser. iconic, honestly. 👑
To illustrate this feature, we'll design a pattern that matches if a string contains a palindrome. Quick definition: a palindrome is a word or a sentence that, while ignoring spaces, interpunctuation and case, reads the same backwards as forwards. We begin by observing that the empty string or a string containing just one word character is a palindrome. Otherwise it must have a word character up front and the same at its end, with another palindrome in between.
/(?: (\w) (?...Here be a palindrome...) \g{-1} | \w? )/xAdding \W* at either end to eliminate what is to be ignored, we already have the full pattern:
my $pp = qr/^(\W* (?: (\w) (?1) \g{-1} | \w? ) \W*)$/ix;
for $s ( "saippuakauppias", "A man, a plan, a canal: Panama!" ){
print "'$s' is a palindrome\n" if $s =~ /$pp/;
}In (?...) both absolute and relative backreferences may be used. The entire pattern can be reinserted with (?R) or (?0). If you prefer to name your groups, you can use (?&name) to recurse into that group. 🌀
🪄 A bit of magic: executing Perl code in a regular expression 🪄
Normally, regexps are a part of Perl expressions. Code evaluation expressions turn that around by allowing arbitrary Perl code to be a part of a regexp. A code evaluation expression is denoted (?{code}), with code a string of Perl statements. not me falling in love with regex magic 😭
⚠️ Be warned that this feature is considered experimental, and may be changed without notice.
Code expressions are zero-width assertions, and the value they return depends on their environment. There are two possibilities: either the code expression is used as a conditional in a conditional expression (?(condition)...), or it is not. If the code expression is a conditional, the code is evaluated and the result (i.e., the result of the last statement) is used to determine truth or falsehood. If the code expression is not used as a conditional, the assertion always evaluates true and the result is put into the special variable $^R. The variable $^R can then be used in code expressions later in the regexp. Here are some silly examples:
$x = "abcdef";
$x =~ /abc(?{print "Hi Mom!";})def/; # matches,
# prints 'Hi Mom!'
$x =~ /aaa(?{print "Hi Mom!";})def/; # doesn't match,
# no 'Hi Mom!'Pay careful attention to the next example:
$x =~ /abc(?{print "Hi Mom!";})ddd/; # doesn't match,
# no 'Hi Mom!'
# but why not?At first glance, you'd think that it shouldn't print, because obviously the ddd isn't going to match the target string. But look at this example:
$x =~ /abc(?{print "Hi Mom!";})[dD]dd/; # doesn't match,
# but _does_ printHmm. What happened here? If you've been following along, you know that the above pattern should be effectively (almost) the same as the last one; enclosing the d in a character class isn't going to change what it matches. So why does the first not print while the second one does? 🤔
The answer lies in the optimizations the regex engine makes. In the first case, all the engine sees are plain old characters (aside from the ?{} construct). It's smart enough to realize that the string 'ddd' doesn't occur in our target string before actually running the pattern through. But in the second case, we've tricked it into thinking that our pattern is more complicated. It takes a look, sees our character class, and decides that it will have to actually run the pattern to determine whether or not it matches, and in the process of running it hits the print statement before it discovers that we don't have a match. 💀
To take a closer look at how the engine does optimizations, see the section "Pragmas and debugging" below.
More fun with ?{}:
$x =~ /(?{print "Hi Mom!";})/; # matches,
# prints 'Hi Mom!'
$x =~ /(?{$c = 1;})(?{print "$c";})/; # matches,
# prints '1'
$x =~ /(?{$c = 1;})(?{print "$^R";})/; # matches,
# prints '1'The bit of magic mentioned in the section title occurs when the regexp backtracks in the process of searching for a match. If the regexp backtracks over a code expression and if the variables used within are localized using local, the changes in the variables produced by the code expression are undone! Thus, if we wanted to count how many times a character got matched inside a group, we could use, e.g.,
$x = "aaaa";
$count = 0; # initialize 'a' count
$c = "bob"; # test if $c gets clobbered
$x =~ /(?{local $c = 0;}) # initialize count
( a # match 'a'
(?{local $c = $c + 1;}) # increment count
)* # do this any number of times,
aa # but match 'aa' at the end
(?{$count = $c;}) # copy local $c var into $count
/x;
print "'a' count is $count, \$c variable is '$c'\n";This prints
'a' count is 2, $c variable is 'bob'If we replace the (?{local $c = $c + 1;}) with (?{$c = $c + 1;}), the variable changes are not undone during backtracking, and we get
'a' count is 4, $c variable is 'bob'Note that only localized variable changes are undone. Other side effects of code expression execution are permanent. Thus
$x = "aaaa";
$x =~ /(a(?{print "Yow\n";}))*aa/;produces
Yow
Yow
Yow
YowThe result $^R is automatically localized, so that it will behave properly in the presence of backtracking. ✨
This example uses a code expression in a conditional to match a definite article, either 'the' in English or 'der|die|das' in German:
$lang = 'DE'; # use German
...
$text = "das";
print "matched\n"
if $text =~ /(?(?{
$lang eq 'EN'; # is the language English?
})
the | # if so, then match 'the'
(der|die|das) # else, match 'der|die|das'
)
/xi;Note that the syntax here is (?(?{...})yes-regexp|no-regexp), not (?((?{...}))yes-regexp|no-regexp). In other words, in the case of a code expression, we don't need the extra parentheses around the conditional. 🫶
If you try to use code expressions with interpolating variables, Perl may surprise you:
$bar = 5;
$pat = '(?{ 1 })';
/foo(?{ $bar })bar/; # compiles ok, $bar not interpolated
/foo(?{ 1 })$bar/; # compile error!
/foo${pat}bar/; # compile error!
$pat = qr/(?{ $foo = 1 })/; # precompile code regexp
/foo${pat}bar/; # compiles okOkay so picture this, bestie: you've got a regexp with (1) code expressions and interpolating variables, or (2) a variable that interpolates a code expression. Perl looks at that and is like "absolutely not, that's an error." But 👀 if the code expression is precompiled into a variable? Then interpolating is fine. And you're probably sitting there like... okay but why is it an error in the first place?
The reason is security. Variable interpolation + code expressions together = a whole risk situation. It's dangerous because so many programmers writing search engines just take user input and plug it straight into a regexp, no thoughts, head empty:
$regexp = <>; # read user-supplied regexp
$chomp $regexp; # get rid of possible newline
$text =~ /$regexp/; # search $text for the $regexpSo if $regexp happens to contain a code expression, the user could then execute arbitrary Perl code. Like, some joker could search for "system('rm -rf *');" and just erase your files. 😭 In that sense, the combo of interpolation and code expressions taints your regexp. So by default, using both interpolation and code expressions in the same regexp is not allowed. If you're genuinely not concerned about malicious users, you can bypass this security check by invoking use re 'eval':
use re 'eval'; # throw caution out the door
$bar = 5;
$pat = '(?{ 1 })';
/foo(?{ 1 })$bar/; # compiles ok
/foo${pat}bar/; # compiles okAnother form of code expression is the pattern code expression. It's like a regular code expression except that — plot twist — the result of the code evaluation gets treated as a regular expression and matched immediately. A simple example:
$length = 5;
$char = 'a';
$x = 'aaaaabb';
$x =~ /(??{$char x $length})/x; # matches, there are 5 of 'a'And this final example has both ordinary and pattern code expressions. It detects whether a binary string 1101010010001... has a Fibonacci spacing 0,1,1,2,3,5,... of the 1's:
$x = "1101010010001000001";
$z0 = ''; $z1 = '0'; # initial conditions
print "It is a Fibonacci sequence\n"
if $x =~ /^1 # match an initial '1'
(?:
((??{ $z0 })) # match some '0'
1 # and then a '1'
(?{ $z0 = $z1; $z1 .= $^N; })
)+ # repeat as needed
$ # that is all there is
/x;
printf "Largest sequence matched was %d\n", length($z1)-length($z0);Remember that $^N is set to whatever was matched by the last completed capture group. This prints:
It is a Fibonacci sequence
Largest sequence matched was 5Ha! Try that with your garden variety regexp package... 💅
Note that the variables $z0 and $z1 are not substituted when the regexp is compiled, the way ordinary variables outside a code expression are. Instead, the code expressions are evaluated when Perl encounters them during the search for a match. 🔥
The regexp without the //x modifier is:
/^1(?:((??{ $z0 }))1(?{ $z0 = $z1; $z1 .= $^N; }))+$/which shows that spaces are still possible in the code parts. That said, when you're working with code and conditional expressions, the extended form of regexps is almost necessary for creating and debugging them. Trust. 🫶
Backtracking control verbs
Perl 5.10 introduced a bunch of control verbs meant to give you detailed control over the backtracking process — directly influencing the regexp engine and giving you monitoring techniques. ⚠️ Heads up though: all the features in this group are experimental and subject to change or removal in a future version of Perl, so if you're interested, go read "Special Backtracking Control Verbs" in perlre for the detailed description.
Below is just one example, illustrating the control verb (*FAIL), which may be abbreviated as (*F). If you insert it in a regexp it will cause it to fail, just like it would at some mismatch between the pattern and the string. Processing of the regexp continues the way it would after any "normal" failure — so, for instance, the next position in the string or another alternative gets tried. And since failing to match doesn't preserve capture groups or produce results, you may need to use this in combination with embedded code:
%count = ();
"supercalifragilisticexpialidoceous" =~
/([aeiou])(?{ $count{$1}++; })(*FAIL)/i;
printf "%3d '%s'\n", $count{$_}, $_ for (sort keys %count);The pattern begins with a class matching a subset of letters. Whenever this matches, a statement like $count{'a'}++; is executed, incrementing that letter's counter. Then (*FAIL) does exactly what it says, and the regexp engine proceeds according to the book: as long as the end of the string hasn't been reached, the position gets advanced before looking for another vowel. So match or no match makes no difference, and the regexp engine proceeds until the entire string has been inspected. (It's remarkable that an alternative solution using something like
$count{lc($_)}++ for split('', "supercalifragilisticexpialidoceous");
printf "%3d '%s'\n", $count2{$_}, $_ for ( qw{ a e i o u } );is considerably slower.)
Pragmas and debugging
Speaking of debugging 🐛 — there are several pragmas available to control and debug regexps in Perl. We already met one in the previous section, use re 'eval';, which lets variable interpolation and code expressions coexist in a regexp. The other pragmas are:
use re 'taint';
$tainted = <>;
@parts = ($tainted =~ /(\w+)\s+(\w+)/; # @parts is now taintedThe taint pragma causes any substrings from a match with a tainted variable to be tainted as well. This is not normally the case, since regexps are often used to extract the safe bits from a tainted variable. Use taint when you're not extracting safe bits, but doing some other processing. Both taint and eval pragmas are lexically scoped, meaning they're in effect only until the end of the block enclosing the pragmas.
use re '/m'; # or any other flags
$multiline_string =~ /^foo/; # /m is impliedThe re '/flags' pragma (introduced in Perl 5.14) turns on the given regular expression flags until the end of the lexical scope. See "re/"'/flags' mode"" for more detail.
use re 'debug';
/^(.*)$/s; # output debugging info
use re 'debugcolor';
/^(.*)$/s; # output debugging info in living colorThe global debug and debugcolor pragmas let you get detailed debugging info about regexp compilation and execution. debugcolor is the same as debug, except the debugging information is displayed in color on terminals that can display termcap color sequences. 🌈 Here's example output:
% perl -e 'use re "debug"; "abc" =~ /a*b+c/;'
Compiling REx `a*b+c'
size 9 first at 1
1: STAR(4)
2: EXACT <a>(0)
4: PLUS(7)
5: EXACT <b>(0)
7: EXACT <c>(9)
9: END(0)
floating `bc' at 0..2147483647 (checking floating) minlen 2
Guessing start of match, REx `a*b+c' against `abc'...
Found floating substr `bc' at offset 1...
Guessed: match at offset 0
Matching REx `a*b+c' against `abc'
Setting an EVAL scope, savestack=3
0 <> <abc> | 1: STAR
EXACT <a> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
1 <a> <bc> | 4: PLUS
EXACT <b> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
2 <ab> <c> | 7: EXACT <c>
3 <abc> <> | 9: END
Match successful!
Freeing REx: `a*b+c'If you've gotten this far into the tutorial, you can probably guess what the different parts of the debugging output are telling you. 👑 The first part
Compiling REx `a*b+c'
size 9 first at 1
1: STAR(4)
2: EXACT <a>(0)
4: PLUS(7)
5: EXACT <b>(0)
7: EXACT <c>(9)
9: END(0)describes the compilation stage. STAR(4) means there's a starred object — in this case 'a' — and if it matches, goto line 4, i.e., PLUS(7). The middle lines describe some heuristics and optimizations performed before a match:
floating `bc' at 0..2147483647 (checking floating) minlen 2
Guessing start of match, REx `a*b+c' against `abc'...
Found floating substr `bc' at offset 1...
Guessed: match at offset 0Then the match is executed and the remaining lines describe the process:
Matching REx `a*b+c' against `abc'
Setting an EVAL scope, savestack=3
0 <> <abc> | 1: STAR
EXACT <a> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
1 <a> <bc> | 4: PLUS
EXACT <b> can match 1 times out of 32767...
Setting an EVAL scope, savestack=3
2 <ab> <c> | 7: EXACT <c>
3 <abc> <> | 9: END
Match successful!
Freeing REx: `a*b+c'Each step is of the form n <x> <y>, with <x> the part of the string matched and <y> the part not yet matched. The | 1: STAR says Perl is at line number 1 in the compilation list above. See "Debugging Regular Expressions" in perldebguts for much more detail. 📚
An alternative method of debugging regexps is to embed print statements right inside the regexp. This gives you a blow-by-blow account of the backtracking in an alternation:
"that this" =~ m@(?{print "Start at position ", pos, "\n";})
t(?{print "t1\n";})
h(?{print "h1\n";})
i(?{print "i1\n";})
s(?{print "s1\n";})
|
t(?{print "t2\n";})
h(?{print "h2\n";})
a(?{print "a2\n";})
t(?{print "t2\n";})
(?{print "Done at position ", pos, "\n";})
@x;prints
Start at position 0
t1
h1
t2
h2
a2
t2
Done at position 4🚨 BUGS 🚨
Code expressions, conditional expressions, and independent expressions are experimental. Don't use them in production code. Not yet, bestie. 🙈
💖 SEE ALSO 💖
okay so bestie, real talk: this whole thing you just read is just a tutorial. like, a vibe introduction. for the full ✨ saga ✨ of Perl regular expressions, you gotta go read the perlre regular expressions reference page. that's the main character. this is the trailer.
if you want the tea on the matching m// operator and the substitution s/// operator, they're documented in "Regexp Quote-Like Operators" in perlop. and for everything about the split operation, go check out "split" in perlfunc.
and if you're looking for an excellent all-around resource on the care and feeding of regular expressions (and honestly, they do require feeding 🍓), there's the book Mastering Regular Expressions by Jeffrey Friedl — published by O'Reilly, ISBN 1556592-257-3. no cap, it's iconic.
🎀 AUTHOR AND COPYRIGHT 🎀
Copyright (c) 2000 Mark Kvale All rights reserved.
This document may be distributed under the same terms as Perl itself.
Acknowledgments 🌸
the inspiration for the stop codon DNA example? it came from the ZIP code example in chapter 7 of Mastering Regular Expressions. slay, honestly.
and the author wants to give a big thank you to Jeff Pinyan, Andrew Johnson, Peter Haworth, Ronald J Kimball, and Joe Smith for all their helpful comments. 🫶 absolute legends, every single one.
and that's a wrap bestie!! 😭💖 you made it through the whole tutorial. go forth and regex responsibly. 👑
okay so this gorgeous little page is perl v5.14.2, dated 2013-07-18, and it's PERLRETUT(1) — periodt. now go write some regex that actually works on the first try (we both know you won't, but the audacity is iconic). 💅✨🌈
📦 where this page comes from
- alpine 3.24: bitkeeper-doc 7.3.3-r1 · Apache-2.0
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on : a language model's rewrite of the original, under its terms