regsub(n)

✨ regsub(n) ✨

okay so bestie, welcome to regsub — a built-in command in the Tcl family, and literally the moment your regex era begins. She does one thing and she does it gorgeously: she goes looking for pattern matches and swaps them out. 👑

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🌸 NAME 🌸

regsub — Perform substitutions based on regular expression pattern matching 💅

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🎀 SYNOPSIS 🎀

regsub ?switches? exp string subSpec ?varName?

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🦋 DESCRIPTION 🦋

So here's the tea. regsub takes the regular expression exp and matches it against string. Then it either:

• copies string into the variable whose name you handed over as varName, or
• just returns string if you didn't pass a varName at all.

(How regular expression matching actually works is a whole other saga — it lives in the re_syntax reference page. Go read her when you're ready. 🫶)

Now, if there's a match, then while it's copying string over into varName (or into the result of this command if there's no varName), the part of string that matched exp gets replaced with subSpec. Iconic behaviour honestly.

Here's the substitution cheat sheet, no cap:

• If subSpec contains a & or a \0, that bit gets replaced in the substitution with the portion of string that matched exp. ✨
• If subSpec contains a \n, where n is a digit between 1 and 9, it gets replaced with the portion of string that matched the n'th parenthesized subexpression of exp. 🎯
• You can stack additional backslashes in subSpec to stop &, \0, \n and backslashes from being interpreted specially.

⚠️ Okay genuine warning time, bestie: backslashes in subSpec tend to interact badly with the Tcl parser's own use of backslashes. It's generally safest to wrap subSpec in braces if it includes backslashes. Do not skip this. 😭

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✨ OPTIONS ✨

If the initial arguments to regsub start with -, they get treated as switches. These are the switches currently supported:

• -all — Every range in string that matches exp gets found, and the substitution is performed for each one of those ranges. Without this switch, only the first matching range gets found and substituted (basic behaviour, lowkey). If -all is specified, then & and \n sequences are handled for each substitution using the information from the corresponding match. 🔥
• -command — This changes how subSpec is handled: it's no longer treated as a template for a substitution string, and the substrings & and \n no longer have any special meaning. Instead, subSpec must be a command prefix, that is, a non-empty list. The substring of string that matches exp, and then each substring that matches each capturing sub-RE within exp, get appended as additional elements to that list. (The items appended to the list are much like what regexp -inline would return, iykyk.) The completed list is then evaluated as a Tcl command, and the result of that command is the substitution string. Any error or exception from that command evaluation becomes an error or exception from the regsub command. 💀
  • If -all is not also given, the command callback will be invoked at most once (exactly when the regular expression matches).
  • If -all is given, the command callback will be invoked for each matched location, in sequence.
  • The exact location indices that matched are not made available to the script.
  • See EXAMPLES below for illustrative cases. 👀
• -expanded — Enables use of the expanded regular expression syntax, where whitespace and comments are ignored. This is the same as specifying the (?x) embedded option (see the re_syntax manual page).
• -line — Enables newline-sensitive matching. By default, newline is a completely ordinary character with no special meaning. With this flag, [^ bracket expressions and . never match newline, ^ matches an empty string after any newline in addition to its normal function, and $ matches an empty string before any newline in addition to its normal function. This flag is equivalent to specifying both -linestop and -lineanchor, or the (?n) embedded option (see the re_syntax manual page).
• -linestop — Changes the behavior of [^ bracket expressions and . so that they stop at newlines. This is the same as specifying the (?p) embedded option (see the re_syntax manual page).
• -lineanchor — Changes the behavior of ^ and $ (the "anchors") so they match the beginning and end of a line respectively. This is the same as specifying the (?w) embedded option (see the re_syntax manual page).
• -nocase — Upper-case characters in string will be converted to lower-case before matching against exp. However, substitutions specified by subSpec use the original unconverted form of string. 🍓
• -start index — Specifies a character index offset into the string to start matching the regular expression at. The index value is interpreted in the same manner as the index argument to string index. When using this switch, ^ will not match the beginning of the line, and \A will still match the start of the string at index. index will be constrained to the bounds of the input string.
• -- — Marks the end of switches. The argument following this one will be treated as exp even if it starts with a -.

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And now the return value moment: if varName is supplied, the command returns a count of the number of matching ranges that were found and replaced. Otherwise, the string after replacement is returned. 🌈

See the manual entry for regexp for the deets on how regular expressions get interpreted.

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💖 EXAMPLES 💖

Replace (in the string in variable string) every instance of foo which is a word by itself with bar:

regsub -all {\mfoo\M} $string bar string

or (using the "basic regular expression" syntax):

regsub -all {(?b)\<foo\>} $string bar string

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Insert double-quotes around the first instance of the word interesting, however it is capitalized:

regsub -nocase {\yinteresting\y} $string {"&"} string

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Convert all non-ASCII and Tcl-significant characters into \u escape sequences by using regsub and subst in combination:

# This RE is just a character class for almost everything "bad"
set RE {[][{};#\\\$ \r\t\u0080-\uffff]}

# We will substitute with a fragment of Tcl script in brackets
set substitution {[format \\\\u%04x [scan "\\&" %c]]}

# Now we apply the substitution to get a subst-string that
# will perform the computational parts of the conversion. Note
# that newline is handled specially through string map since
# backslash-newline is a special sequence.
set quoted [subst [string map {\n {\\u000a}} \
        [regsub -all $RE $string $substitution]]]

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The above operation can be done using regsub -command instead, which is often faster. (A full pre-computed string map would be faster still, but the cost of computing the map for a transformation as complex as this can be quite large.) 🫶

# This RE is just a character class for everything "bad"
set RE {[][{};#\\\$\s\u0080-\uffff]}

# This encodes what the RE described above matches
proc encodeChar {ch} {
    # newline is handled specially since backslash-newline is a
    # special sequence.
    if {$ch eq "\n"} {
        return "\\u000a"
    }
    # No point in writing this as a one-liner
    scan $ch %c charNumber
    format "\\u%04x" $charNumber
}

set quoted [regsub -all -command $RE $string encodeChar]

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Decoding a URL-encoded string using regsub -command, a lambda term and the apply command:

# Match one of the sequences in a URL-encoded string that needs
# fixing, converting + to space and %XX to the right character
# (e.g., %7e becomes ~)
set RE {(\+)|%([0-9A-Fa-f]{2})}

# Note that -command uses a command prefix, not a command name
set decoded [regsub -all -command $RE $string {apply {{- p h} {
    # + is a special case; handle directly
    if {$p eq "+"} {
        return " "
    }
    # convert hex to a char
    scan $h %x charNumber
    format %c $charNumber
}}}]

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The -command option can also be useful for restricting the range of commands such as string totitle:

set message "the quIck broWn fOX JUmped oVer the laZy dogS..."
puts [regsub -all -command {\w+} $message {string totitle}]
# → The Quick Brown Fox Jumped Over The Lazy Dogs..

She really ate that one. 💅

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🧚 SEE ALSO 🧚

regexp(n), re_syntax(n), subst(n), string(n)

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⭐ KEYWORDS ⭐

match, pattern, quoting, regular expression, substitution

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and that's the whole vibe, bestie 🦋 — this page is from the Tcl Built-In Commands manual, version 8.3, dated regsub(n), so go forth and substitute responsibly. mwah 💜