PERLREBACKSLASH(1)

💅 PERLREBACKSLASH 💅

okay so you're here because you want to know about all the backslash and escape sequences in Perl regex? periodt. this page is perlrebackslash from the Perl Programmers Reference Guide. it's giving "quick reference but make it iconic". 💖

📛 NAME

perlrebackslash — Perl Regular Expression Backslash Sequences and Escapes. no cap. 🎀

📖 DESCRIPTION

the top-level tea about Perl regular expressions lives in perlre. this doc right here? it describes all backslash and escape sequences. after explaining what the backslash is even doing, it lists every sequence that has a special meaning in Perl regex (alphabetical order, slay), then describes each one. most sequences are described in detail in other docs, but the main character energy of this doc is to be a quick reference guide describing all backslash and escape sequences. iykyk. ✨

🪄 The backslash

in a regex, the backslash can do one of two things: it either takes away the special meaning of the character after it (like \| matches a literal vertical bar, not an alternation), or it's the start of a backslash or escape sequence. 💅

the rules are lowkey simple: if the character after the backslash is an ASCII punctuation (non-word) character — that's anything that's not a letter, digit, or underscore — then the backslash just removes any special meaning from the character after it.

if the character after the backslash is an ASCII letter or digit, then the sequence may be special. if it is, it's listed below. a few letters haven't been used yet, so escaping them with a backslash doesn't make them special. a future version of Perl might assign them a special meaning, so if you have warnings on, Perl throws a warning if you use such a sequence. [1].

but here's the guarantee: backslash or escape sequences never have a punctuation character following the backslash, not now, not in any future Perl 5. so it's totally safe to put a backslash in front of a non-word character. 🫶

note that the backslash itself is special. if you want to match a backslash, you have to escape it with another backslash: "/\\/" matches a single backslash. iconic.

[1] there's one exception. if you use an alphanumeric character as the delimiter of your pattern (which you probably shouldn't do for readability reasons, bestie), you have to escape the delimiter if you want to match it. Perl won't warn then. see also "Gory details of parsing quoted constructs" in perlop. 😭

📜 All the sequences and escapes

those not usable within a bracketed character class (like [\da-z]) are marked as "Not in []." got it? let's go. 🏃‍♀️

\000 — octal escape sequence. see also \o{}.
\1 — absolute backreference. Not in [].
\a — alarm or bell.
\A — beginning of string. Not in [].
\b{}, \b — boundary. (\b is a backspace in []).
\B{}, \B — not a boundary. Not in [].
\cX — control-X.
\d — match any digit character.
\D — match any character that isn't a digit.
\e — escape character.
\E — turn off \Q, \L and \U processing. Not in [].
\f — form feed.
\F — foldcase till \E. Not in [].
\g{}, \g1 — named, absolute or relative backreference. Not in [].
\G — pos assertion. Not in [].
\h — match any horizontal whitespace character.
\H — match any character that isn't horizontal whitespace.
\k{}, \k<>, \k'' — named backreference. Not in [].
\K — keep the stuff left of \K. Not in [].
\l — lowercase next character. Not in [].
\L — lowercase till \E. Not in [].
\n — (logical) newline character.
\N — match any character but newline. Not in [].
\N{} — named or numbered (Unicode) character or sequence.
\o{} — octal escape sequence.
\p{}, \pP — match any character with the given Unicode property.
\P{}, \PP — match any character without the given property.
\Q — quote (disable) pattern metacharacters till \E. (also called "escape".) Not in [].
\r — return character.
\R — generic new line. Not in [].
\s — match any whitespace character.
\S — match any character that isn't a whitespace.
\t — tab character.
\u — titlecase next character. Not in [].
\U — uppercase till \E. Not in [].
\v — match any vertical whitespace character.
\V — match any character that isn't vertical whitespace.
\w — match any word character.
\W — match any character that isn't a word character.
\x{}, \x00 — hexadecimal escape sequence.
\X — Unicode "extended grapheme cluster". Not in [].
\z — end of string. Not in [].
\Z — end of string. Not in [].

🔤 Character Escapes

🔒 Fixed characters

a handful of characters have a dedicated character escape. the table below shows them, along with their ASCII code points (in decimal and hex), their ASCII name, the control escape on ASCII platforms, and a short description. (for EBCDIC platforms, see "OPERATOR DIFFERENCES" in perlebcdic.) 💖

 Seq.  Code Point  ASCII   Cntrl   Description.
       Dec    Hex
  \a     7     07    BEL    \cG    alarm or bell
  \b     8     08     BS    \cH    backspace [1]
  \e    27     1B    ESC    \c[    escape character
  \f    12     0C     FF    \cL    form feed
  \n    10     0A     LF    \cJ    line feed [2]
  \r    13     0D     CR    \cM    carriage return
  \t     9     09    TAB    \cI    tab

[1] \b is the backspace character only inside a character class. outside a character class, \b alone is a word-character/non-word-character boundary, and \b{} is some other type of boundary. 🤯

[2] \n matches a logical newline. Perl converts between \n and your OS's native newline character when reading from or writing to text files. ✨

🌰 Example

$str =~ /\t/;   # Matches if $str contains a (horizontal) tab.

🎛️ Control characters

\c is used to denote a control character. the character following \c determines the value of the construct. for example, the value of \cA is chr(1), and the value of \cb is chr(2), etc. the gory details are in "Regexp Quote-Like Operators" in perlop. a complete list of what chr(1), etc. means for ASCII and EBCDIC platforms is in "OPERATOR DIFFERENCES" in perlebcdic. 💅

note that \c\ alone at the end of a regular expression (or doubled-quoted string) is not valid. the backslash must be followed by another character. that is, \c\X means "chr(28) . 'X'" for all characters X. 😭

to write platform-independent code, you must use \N{NAME} instead, like \N{ESCAPE} or \N{U+001B}, see charnames. 🫶

mnemonic: control character. 👑

example:

$str =~ /\cK/;  # Matches if $str contains a vertical tab (control-K).

🏷️ Named or numbered characters and character sequences

Unicode characters have a Unicode name and a numeric code point (ordinal) value. use the \N{} construct to specify a character by either of these values. certain sequences of characters also have names. 🌈

to specify by name, the name of the character or character sequence goes between the curly braces. ✨

to specify a character by Unicode code point, use the form \N{U+code point}, where code point is a number in hexadecimal that gives the code point that Unicode has assigned to the desired character. it's customary but not required to use leading zeros to pad the number to 4 digits. thus \N{U+0041} means "LATIN CAPITAL LETTER A", and you will rarely see it written without the two leading zeros. \N{U+0041} means "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41). 🤯

blanks may freely be inserted adjacent to but within the braces enclosing the name or code point. so \N{ U+0041 } is perfectly legal. 💖

it's even possible to give your own names to characters and character sequences by using the charnames module. these custom names are lexically scoped, so a given code point may have different names in different scopes. the name used is what is in effect at the time the \N{} is expanded. for patterns in double-quotish context, that means at the time the pattern is parsed. but for patterns that are delimited by single quotes, the expansion is deferred until pattern compilation time, which may very well have a different charnames translator in effect. 😵‍💫

(there is an expanded internal form that you may see in debug output: \N{U+code point.code point...}. the ... means any number of these code points separated by dots. this represents the sequence formed by the characters. this is an internal form only, subject to change, and you should not try to use it yourself.) 🙈

mnemonic: named character. 💅

note that a character or character sequence expressed as a named or numbered character is considered a character without special meaning by the regex engine, and will match "as is". no cap.

example:

$str =~ /\N{THAI CHARACTER SO SO}/;  # Matches the Thai SO SO character

use charnames 'Cyrillic';            # Loads Cyrillic names.
$str =~ /\N{ZHE}\N{KA}/;             # Match "ZHE" followed by "KA".

🐙 Octal escapes

there are two forms of octal escapes. each is used to specify a character by its code point specified in base 8. 🎱

one form, available starting in Perl 5.14, looks like \o{...}, where the dots represent one or more octal digits. it can be used for any Unicode character. ✨

it was introduced to avoid the potential problems with the other form, available in all Perls. that form consists of a backslash followed by three octal digits. one problem with this form is that it can look exactly like an old-style backreference (see "Disambiguation rules between old-style octal escapes and backreferences" below.) you can avoid this by making the first of the three digits always a zero, but that makes \077 the largest code point specifiable. 😭

in some contexts, a backslash followed by two or even one octal digits may be interpreted as an octal escape, sometimes with a warning, and because of some bugs, sometimes with surprising results. also, if you are creating a regex out of smaller snippets concatenated together, and you use fewer than three digits, the beginning of one snippet may be interpreted as adding digits to the ending of the snippet before it. see "Absolute referencing" for more discussion and examples of the snippet problem. 💀

note that a character expressed as an octal escape is considered a character without special meaning by the regex engine, and will match "as is". 🫶

okay so, to summarize the whole octal situation: the "\o{}" form is always safe to use, no thoughts, no caveats. The other form is safe for code points through \077 — but ONLY when you specify them with exactly three digits. Three digits exactly. Not two. Not four. Three. 💅

Mnemonic: 0ctal or octal.

Examples (assuming an ASCII platform):

 $str = "Perl";
 $str =~ /\o{120}/;  # Match, "\120" is "P".
 $str =~ /\120/;     # Same.
 $str =~ /\o{120}+/; # Match, "\120" is "P",
                     # it's repeated at least once.
 $str =~ /\120+/;    # Same.
 $str =~ /P\053/;    # No match, "\053" is "+" and taken literally.
 /\o{23073}/         # Black foreground, white background smiling face.
 /\o{4801234567}/    # Raises a warning, and yields chr(4).
 /\o{ 400}/          # LATIN CAPITAL LETTER A WITH MACRON
 /\o{ 400 }/         # Same. These show blanks are allowed adjacent to
                     # the braces

Disambiguation rules between old-style octal escapes and backreferences

Okay bestie, this is the messy part, and honestly? It's giving chaos. Old-style octal escapes of the "\000" form, when they're outside of bracketed character classes, can potentially clash with old-style backreferences (peep "Absolute referencing" below 👇). Both of them are literally just a backslash followed by numbers. So Perl has to use heuristics to figure out which one you meant. Here's the tea, these are the rules:

If the backslash is followed by a single digit, it's a backreference.
If the first digit following the backslash is a 0, it's an octal escape.
If the number following the backslash is N (in decimal), and Perl has already seen N capture groups, Perl considers this a backreference. Otherwise, it considers it an octal escape. If N has more than three digits, Perl takes only the first three for the octal escape; the rest are matched as is.

 my $pat  = "(" x 999;
    $pat .= "a";
    $pat .= ")" x 999;
 /^($pat)\1000$/;   #  Matches 'aa'; there are 1000 capture groups.
 /^$pat\1000$/;     #  Matches 'a@0'; there are 999 capture groups
                    #  and \1000 is seen as \100 (a '@') and a '0'.

You can force a backreference interpretation always by using the "\g{...}" form. And you can force an octal interpretation always by using the "\o{...}" form, or — for numbers up through \077 (= 63 decimal) — by using three digits, beginning with a "0". Slay, choices. ✨

Hexadecimal escapes

Like octal escapes, there are two forms of hexadecimal escapes, but both start with the sequence "\x". After that comes either exactly two hexadecimal digits forming a number, or a hexadecimal number of arbitrary length surrounded by curly braces. That hexadecimal number is the code point of the character you want to express. 🦋

Note that a character expressed as one of these escapes is considered a character without special meaning by the regex engine, and it'll match "as is". No drama, no metacharacter shenanigans.

Mnemonic: hexadecimal.

Examples (assuming an ASCII platform):

 $str = "Perl";
 $str =~ /\x50/;    # Match, "\x50" is "P".
 $str =~ /\x50+/;   # Match, "\x50" is "P", it is repeated at least once
 $str =~ /P\x2B/;   # No match, "\x2B" is "+" and taken literally.

 /\x{2603}\x{2602}/ # Snowman with an umbrella.
                    # The Unicode character 2603 is a snowman,
                    # the Unicode character 2602 is an umbrella.
 /\x{263B}/         # Black smiling face.
 /\x{263b}/         # Same, the hex digits A - F are case insensitive.
 /\x{ 263b }/       # Same, showing optional blanks adjacent to the
                    # braces

✨ Modifiers ✨

A bunch of backslash sequences are all about changing the character — or characters — that follow them. "\l" will lowercase the character following it, while "\u" will uppercase (or, more accurately, titlecase) the character following it. They give you functionality similar to the functions "lcfirst" and "ucfirst". 🎀

Want to uppercase or lowercase several characters? That's what "\L" and "\U" are for — they'll lowercase/uppercase everything following them, until either the end of the pattern or the next occurrence of "\E", whichever comes first. Basically the same energy as the functions "lc" and "uc".

"\Q" is the quote-or-escape girly — it disables pattern metacharacters up to the next "\E" or the end of the pattern. "\Q" adds a backslash to any character that could have special meaning to Perl. In the ASCII range, it quotes every character that isn't a letter, digit, or underscore. For the details on what gets quoted for non-ASCII code points, see "quotemeta" in perlfunc. Using this guarantees that any character between "\Q" and "\E" gets matched literally, and never gets interpreted as a metacharacter by the regex engine. 🫶

"\F" casefolds all characters following it, up to the next "\E" or the end of the pattern. Same vibe as the "fc" function.

Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.

Examples:

 $sid     = "sid";
 $greg    = "GrEg";
 $miranda = "(Miranda)";
 $str     =~ /\u$sid/;        # Matches 'Sid'
 $str     =~ /\L$greg/;       # Matches 'greg'
 $str     =~ /\Q$miranda\E/;  # Matches '(Miranda)', as if the pattern
                              #   had been written as /\(Miranda\)/

🌈 Character classes 🌈

Perl regular expressions have a huge range of character classes, and some of them are written as a backslash sequence. We'll keep it brief here — the full deets live in perlrecharclass.

"\w" is a character class that matches any single word character (letters, digits, Unicode marks, and connector punctuation (like the underscore)). "\d" matches any decimal digit, while "\s" matches any whitespace character. New in perl 5.10.0 are "\h" and "\v", which match horizontal and vertical whitespace characters. 👑

The exact set of characters matched by "\d", "\s", and "\w" varies depending on various pragma and regular expression modifiers. You can restrict the match to the ASCII range by using the "/a" regular expression modifier. See perlrecharclass.

The uppercase variants ("\W", "\D", "\S", "\H", and "\V") match, respectively, any character that isn't a word character, digit, whitespace, horizontal whitespace, or vertical whitespace. It's giving negation.

Mnemonics: word, digit, space, horizontal, vertical.

Unicode classes

"\pP" (where "P" is a single letter) and "\p{Property}" match a character that matches the given Unicode property. Properties include things like "letter", or "thai character". Capitalize it — "\PP" and "\P{Property}" — and now you're matching a character that doesn't match that Unicode property. 💜

For more details, see "Backslash sequences" in perlrecharclass and "Unicode Character Properties" in perlunicode.

Mnemonic: property.

💖 Referencing 💖

If capturing parenthesis are used in a regular expression, we can refer to the part of the source string that got matched, and match exactly the same thing. There are three ways of referring to such a backreference: absolutely, relatively, and by name. Pick your fighter.

Absolute referencing

Either "\gN" (starting in Perl 5.10.0), or "\N" (old-style), where N is a positive (unsigned) decimal number of any length, is an absolute reference to a capturing group.

N refers to the Nth set of parentheses, so "\gN" refers to whatever has been matched by that set of parentheses. So "\g1" refers to the first capture group in the regex.

The "\gN" form can be written equivalently as "\g{N}", which dodges ambiguity when you're building a regex by concatenating shorter strings. Otherwise, if you had a regex "qr/$x$y/", and $x contained "\g1", and $y contained "37", you'd end up with "/\g137/" — which is probably not what you intended. 😭

In the "\N" form, N must not begin with a "0", and there must be at least N capturing groups, or else N is considered an octal escape (but something like "\18" is the same as "\0018"; that is, the octal escape "\001" followed by a literal digit "8").

Mnemonic: group.

Examples:

 /(\w+) \g1/;    # Finds a duplicated word, (e.g. "cat cat").
 /(\w+) \1/;     # Same thing; written old-style.
 /(\w+) \g{1}/;  # Same, using the safer braced notation
 /(\w+) \g{ 1 }/;# Same, showing optional blanks adjacent to the braces
 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").

Relative referencing

"\g-N" (starting in Perl 5.10.0) is for relative addressing. (It can also be written as "\g{-N}".) It refers to the Nth group before the "\g{-N}".

The big advantage here? It makes it so much easier to write patterns with references that can be interpolated into larger patterns — even if that larger pattern also has capture groups in it. No counting from the top, no tears. 🧚

Examples:

 /(A)        # Group 1
  (          # Group 2
    (B)      # Group 3
    \g{-1}   # Refers to group 3 (B)
    \g{-3}   # Refers to group 1 (A)
    \g{ -3 } # Same, showing optional blanks adjacent to the braces
  )
 /x;         # Matches "ABBA".

 my $qr = qr /(.)(.)\g{-2}\g{-1}/;  # Matches 'abab', 'cdcd', etc.
 /$qr$qr/                           # Matches 'ababcdcd'.

Named referencing

"\g{name}" (starting in Perl 5.10.0) lets you back-reference a named capture group, so you never have to think about capture buffer positions ever again. Main character energy, honestly. 💅

To stay compatible with .Net regular expressions, "\g{name}" may also be written as "\k{name}", "\k<name>" or "\k'name'".

And to prevent any ambiguity: name must not start with a digit, and must not contain a hyphen.

Examples:

 /(?<word>\w+) \g{word}/   # Finds duplicated word, (e.g. "cat cat")
 /(?<word>\w+) \k{word}/   # Same.
 /(?<word>\w+) \g{ word }/ # Same, showing optional blanks adjacent to
                           # the braces
 /(?<word>\w+) \k{ word }/ # Same.
 /(?<word>\w+) \k<word>/   # Same.  There are no braces, so no blanks
                           # are permitted
 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
                           # Match a four letter palindrome (e.g.
                           # "ABBA")

🔥 Assertions 🔥

Assertions are conditions that have to be true — but plot twist, they don't actually match parts of the substring. There are six assertions written as backslash sequences.

\A — "\A" only matches at the beginning of the string. If the "/m" modifier isn't used, then "/\A/" is equivalent to "/^/". However, if the "/m" modifier is used, then "/^/" matches internal newlines, but the meaning of "/\A/" isn't changed by the "/m" modifier. "\A" matches at the beginning of the string regardless of whether the "/m" modifier is used. Periodt. 💀

`\z`, `\Z` 🎀

okay so \z and \Z both match at the end of the string. If you're not using the /m modifier, then /\Z/ is literally equivalent to /$/ — like, it matches at the end of the string, or one character before the newline at the end of the string. But if you are using /m, then /$/ matches at internal newlines, whereas the meaning of /\Z/ does not change because of /m. \Z matches at the end of the string (or just before a trailing newline) no matter whether /m is used. no cap.

\z is just like \Z except it does not match before a trailing newline. \z matches at the end of the string only, regardless of any modifiers, and not just before a newline. It's how you anchor your match to the true end of the string under all conditions. periodt. 💅

`\G` ✨

\G is usually only used in combination with the /g modifier. When /g is used and the match happens in scalar context, Perl remembers where in the source string the last match ended, and next time it'll start matching from exactly where it stopped last time. so it's giving continuity.

\G matches the point where the previous match on that string ended, or the beginning of that string if there was no previous match. 👑

Mnemonic: Global.

`\b{}`, `\b`, `\B{}`, `\B` 🦋

\b{...}, available starting in v5.22, matches a boundary (between two characters, or before the first character of the string, or after the final character of the string) based on the Unicode rules for the boundary type you specify inside the braces. The boundary types are listed a few paragraphs down. And \B{...} matches at any place between characters where \b{...} of the same type does not match. opposites, bestie.

\b when it's not immediately followed by a { is available in all Perls. It matches at any place between a word (something matched by \w) and a non-word character (\W). \B, when not immediately followed by a {, matches at any place between characters where \b doesn't match. If you want better word matching for natural language text, peep \b{wb} below. 👀

\b and \B assume there's a non-word character before the beginning and after the end of the source string. So \b will match at the beginning (or end) of the source string if the source string begins (or ends) with a word character. Otherwise, \B will match.

Do not use something like \b=head\d\b and expect it to match the beginning of a line. It literally can't, because for there to be a boundary before the non-word =, there has to be a word character immediately before it. All plain \b and \B boundary determinations look for word characters alone, not for non-word characters and not for string ends.

It might help to understand how \b and \B work by equating them like this:

\b  really means    (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
\B  really means    (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))

In contrast, \b{...} and \B{...} may or may not match at the beginning and end of the line, depending on the boundary type. These implement the Unicode default boundaries, specified in <https://www.unicode.org/reports/tr14/> and <https://www.unicode.org/reports/tr29/>. The boundary types are:

\b{gcb} or \b{g} — this matches a Unicode "Grapheme Cluster Boundary". (Actually Perl always uses the improved "extended" grapheme cluster".) These are explained below under \X. In fact, \X is another way to get the same functionality. It is equivalent to /.+?\b{gcb}/. Use whichever is most convenient for your situation. 🌸
\b{lb} — this matches according to the default Unicode Line Breaking Algorithm (<https://www.unicode.org/reports/tr14/>), as customized in that document (Example 7 of revision 35 <https://www.unicode.org/reports/tr14/tr14-35.html#Example7>) for better handling of numeric expressions.

This is suitable for many purposes, but the Unicode::LineBreak module is available on CPAN and it provides many more features, including customization.

\b{sb} — this matches a Unicode "Sentence Boundary". It's an aid to parsing natural language sentences. It gives good, but imperfect results. For example, it thinks that "Mr. Smith" is two sentences. More details are at <https://www.unicode.org/reports/tr29/>. Also note that it thinks that anything matching \R (except form feed and vertical tab) is a sentence boundary. \b{sb} works with text designed for word-processors which wrap lines automatically for display, but hard-coded line boundaries are considered to be essentially the ends of text blocks (paragraphs really), and hence the ends of sentences. \b{sb} doesn't do well with text containing embedded newlines, like the source text of the document you are reading. Such text needs to be preprocessed to get rid of the line separators before looking for sentence boundaries. Some people view this as a bug in the Unicode standard, and this behavior is quite subject to change in future Perl versions. 🙈
\b{wb} — this matches a Unicode "Word Boundary", but tailored to Perl expectations. This gives better (though not perfect) results for natural language processing than plain \b (without braces) does. For example, it understands that apostrophes can be in the middle of words and that parentheses aren't (see the examples below). More details are at <https://www.unicode.org/reports/tr29/>.

The current Unicode definition of a Word Boundary matches between every white space character. Perl tailors this, starting in version 5.24, to generally not break up spans of white space, just as plain \b has always functioned. This allows \b{wb} to be a drop-in replacement for \b, but with generally better results for natural language processing. (The exception to this tailoring is when a span of white space is immediately followed by something like U+0303, COMBINING TILDE. If the final space character in the span is a horizontal white space, it is broken out so that it attaches instead to the combining character. To be precise, if a span of white space that ends in a horizontal space has the character immediately following it have any of the Word Boundary property values "Extend", "Format" or "ZWJ", the boundary between the final horizontal space character and the rest of the span matches \b{wb}. In all other cases the boundary between two white space characters matches \B{wb}.)

It is important to realize when you use these Unicode boundaries, that you are taking a risk that a future version of Perl which contains a later version of the Unicode Standard will not work precisely the same way as it did when your code was written. These rules are not considered stable and have been somewhat more subject to change than the rest of the Standard. Unicode reserves the right to change them at will, and Perl reserves the right to update its implementation to Unicode's new rules. In the past, some changes have been because new characters have been added to the Standard which have different characteristics than all previous characters, so new rules are formulated for handling them. These should not cause any backward compatibility issues. But some changes have changed the treatment of existing characters because the Unicode Technical Committee has decided that the change is warranted for whatever reason. This could be to fix a bug, or because they think better results are obtained with the new rule. 💀

It is also important to realize that these are default boundary definitions, and that implementations may wish to tailor the results for particular purposes and locales. For example, some languages, such as Japanese and Thai, require dictionary lookup to accurately determine word boundaries.

Mnemonic: boundary.

🌈 Examples 🌈

"cat"   =~ /\Acat/;     # Match.
"cat"   =~ /cat\Z/;     # Match.
"cat\n" =~ /cat\Z/;     # Match.
"cat\n" =~ /cat\z/;     # No match.

"cat"   =~ /\bcat\b/;   # Matches.
"cats"  =~ /\bcat\b/;   # No match.
"cat"   =~ /\bcat\B/;   # No match.
"cats"  =~ /\bcat\B/;   # Match.

while ("cat dog" =~ /(\w+)/g) {
    print $1;           # Prints 'catdog'
}
while ("cat dog" =~ /\G(\w+)/g) {
    print $1;           # Prints 'cat'
}

my $s = "He said, \"Is pi 3.14? (I'm not sure).\"";
print join("|", $s =~ m/ ( .+? \b     ) /xg), "\n";
print join("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
prints
He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"

🍓 Misc 🍓

Here we document the backslash sequences that don't fall in one of the categories above. These are:

\K — this appeared in perl 5.10.0. Anything matched left of \K is not included in $&, and will not be replaced if the pattern is used in a substitution. This lets you write s/PAT1 \K PAT2/REPL/x instead of s/(PAT1) PAT2/${1}REPL/x or s/(?<=PAT1) PAT2/REPL/x.

Mnemonic: Keep.

\N — this feature, available starting in v5.12, matches any character that is not a newline. It's a short-hand for writing [^\n], and is identical to the . metasymbol, except under the /s flag, which changes the meaning of . but not \N. 👑

Note that \N{...} can mean a named or numbered character.

Mnemonic: Complement of \n.

\R — \R matches a generic newline; that is, anything considered a linebreak sequence by Unicode. This includes all characters matched by \v (vertical whitespace), and the multi character sequence \x0D\x0A (carriage return followed by a line feed, sometimes called the network newline; it's the end of line sequence used in Microsoft text files opened in binary mode). \R is equivalent to (?>\x0D\x0A|\v). (The reason it doesn't backtrack is that the sequence is considered inseparable. That means that

 "\x0D\x0A" =~ /^\R\x0A$/   # No match

...it fails, bestie 😭 — because the \R goes ahead and matches the entire string, and it will not backtrack to match just the \x0D. It said "all of it or nothing," and honestly? Respect. 💅

And because \R can match a sequence of more than one character, you literally cannot put it inside a bracketed character class. So /[\R]/ is an error — not a vibe, not a warning, an error. Use \v instead. 🙏

\R was introduced in perl 5.10.0. Ancient icon behavior, fr fr.

Big heads up: this does not respect any locale that might be in effect. It matches according to the platform's native character set. That's just how it is, no notes. ✨

Mnemonic: none really, ngl. \R was picked because PCRE already uses \R, and more importantly because Unicode recommends such a regular expression metacharacter, and suggests \R as its notation. So it's a committee-approved slay. 🫶

🌟 `\X` 🌟

Okay so this one matches a Unicode extended grapheme cluster. 👑

\X matches quite well what normal (non-Unicode-programmer) usage would consider a single character. Let me paint the picture: consider a G with some sort of diacritic mark, such as an arrow. There is no such single character in Unicode — the audacity — but you can compose one by using a G followed by a Unicode "COMBINING UPWARDS ARROW BELOW", and Unicode-aware software would display it as if it were a single character. The way that just works? Obsessed. 🦋

The match is greedy and non-backtracking, so that the cluster is never broken up into smaller components. It's giving loyalty. It's giving commitment. It's giving main character energy that refuses to be subdivided. 💖

🎀 Examples 🎀

Here's the tea, in code form, exactly as written:

 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
 $str =~ s/(.)\K\g1//g;    # Delete duplicated characters.

 "\n"   =~ /^\R$/;         # Match, \n   is a generic newline.
 "\r"   =~ /^\R$/;         # Match, \r   is a generic newline.
 "\r\n" =~ /^\R$/;         # Match, \r\n is a generic newline.

 "P\x{307}" =~ /^\X$/     # \X matches a P with a dot above.

Like... look at those. Every single one of them matching. The \R eating all three flavors of newline? Iconic. The \X grabbing a P with a dot above like it's nothing? Ate. 🍓

And that's a wrap on this page, bestie 🎀✨ — this was perl v5.42.3, dated 2026-08-03, straight out of PERLREBACKSLASH(1). Go forth and backslash responsibly. 💜🦋

📦 where this page comes from

fedora 43: perl-doc 5.42.3-524.fc43 · (GPL-1.0-or-later OR Artistic-1.0-Perl) AND Unicode-3.0 AND LicenseRef-Fedora-UltraPermissive
fedora 44: perl-doc 5.42.3-525.fc44 · (GPL-1.0-or-later OR Artistic-1.0-Perl) AND Unicode-3.0 AND LicenseRef-Fedora-UltraPermissive
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-13: a language model's rewrite of the original, under its terms

PERLREBACKSLASH(1)