PERLRE

# NAME

perlre - Perl regular expressions

# DESCRIPTION

For a description of how to use regular expressions in matching operations, see m// and s/// in the perlop manpage . The matching operations can have various modifiers, some of which relate to the interpretation of the regular expression inside. These are:

        i   Do case-insensitive pattern matching.
        m   Treat string as multiple lines.
        s   Treat string as single line.
        x   Use extended regular expressions.

These are usually written as "the `/x` modifier", even though the delimiter in question might not actually be a slash. In fact, any of these modifiers may also be embedded within the regular expression itself using the new `(?...)` construct. See below.

The `/x` modifier itself needs a little more explanation. It tells the regular expression parser to ignore whitespace that is not backslashed or within a character class. You can use this to break up your regular expression into (slightly) more readable parts. Together with the capability of embedding comments described later, this goes a long way towards making Perl 5 a readable language. See the C comment deletion code in the perlop manpage .

## Regular Expressions

The patterns used in pattern matching are regular expressions such as those supplied in the Version 8 regexp routines. (In fact, the routines are derived (distantly) from Henry Spencer's freely redistributable reimplementation of the V8 routines.) See Version 8 Regular Expressions for details.

In particular the following metacharacters have their standard egrep-ish meanings:

        \   Quote the next metacharacter
        ^   Match the beginning of the line
        .   Match any character (except newline)
        $   Match the end of the line
        |   Alternation
        ()  Grouping
        []  Character class

By default, the "^" character is guaranteed to match only at the beginning of the string, the "$" character only at the end (or before the newline at the end) and Perl does certain optimizations with the assumption that the string contains only one line. Embedded newlines will not be matched by "^" or "$". You may, however, wish to treat a string as a multi-line buffer, such that the "^" will match after any newline within the string, and "$" will match before any newline. At the cost of a little more overhead, you can do this by using the /m modifier on the pattern match operator. (Older programs did this by setting $* , but this practice is deprecated in Perl 5.)

To facilitate multi-line substitutions, the "." character never matches a newline unless you use the `/s` modifier, which tells Perl to pretend the string is a single line--even if it isn't. The `/s` modifier also overrides the setting of $* , in case you have some (badly behaved) older code that sets it in another module.

The following standard quantifiers are recognized:

        *      Match 0 or more times
        +      Match 1 or more times
        ?      Match 1 or 0 times
        {n}    Match exactly n times
        {n,}   Match at least n times
        {n,m}  Match at least n but not more than m times

(If a curly bracket occurs in any other context, it is treated as a regular character.) The "*" modifier is equivalent to `{0,}`, the "+" modifier to `{1,}`, and the "?" modifier to `{0,1}`. There is no limit to the size of n or m, but large numbers will chew up more memory.

By default, a quantified subpattern is "greedy", that is, it will match as many times as possible without causing the rest pattern not to match. The standard quantifiers are all "greedy", in that they match as many occurrences as possible (given a particular starting location) without causing the pattern to fail. If you want it to match the minimum number of times possible, follow the quantifier with a "?" after any of them. Note that the meanings don't change, just the "gravity":

        *?     Match 0 or more times
        +?     Match 1 or more times
        ??     Match 0 or 1 time
        {n}?   Match exactly n times
        {n,}?  Match at least n times
        {n,m}? Match at least n but not more than m times

Since patterns are processed as double quoted strings, the following also work:

        \t          tab
        \n          newline
        \r          return
        \f          form feed
        \v          vertical tab, whatever that is
        \a          alarm (bell)
        \e          escape
        \033        octal char
        \x1b        hex char
        \c[         control char
        \l          lowercase next char
        \u          uppercase next char
        \L          lowercase till \E
        \U          uppercase till \E
        \E          end case modification
        \Q          quote regexp metacharacters till \E

In addition, Perl defines the following:

        \w  Match a "word" character (alphanumeric plus "_")
        \W  Match a non-word character
        \s  Match a whitespace character
        \S  Match a non-whitespace character
        \d  Match a digit character
        \D  Match a non-digit character

Note that `\w` matches a single alphanumeric character, not a whole word. To match a word you'd need to say `\w+`. You may use `\w`, `\W`, `\s`, `\S`, `\d` and `\D` within character classes (though not as either end of a range).

Perl defines the following zero-width assertions:

        \b  Match a word boundary
        \B  Match a non-(word boundary)
        \A  Match only at beginning of string
        \Z  Match only at end of string
        \G  Match only where previous m//g left off

A word boundary (`\b`) is defined as a spot between two characters that has a `\w` on one side of it and and a `\W` on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a `\W`. (Within character classes `\b` represents backspace rather than a word boundary.) The `\A` and `\Z` are just like "^" and "$" except that they won't match multiple times when the `/m` modifier is used, while "^" and "$" will match at every internal line boundary.

When the bracketing construct `( ... )` is used, <digit> matches the digit'th substring. (Outside of the pattern, always use "$" instead of "\" in front of the digit. The scope of <digit> (and $` , `$&`, and `$')` extends to the end of the enclosing BLOCK or eval string, or to the next pattern match with subexpressions. If you want to use parentheses to delimit subpattern (e.g. a set of alternatives) without saving it as a subpattern, follow the ( with a ?. The <digit> notation sometimes works outside the current pattern, but should not be relied upon.) You may have as many parentheses as you wish. If you have more than 9 substrings, the variables $10, $11, ... refer to the corresponding substring. Within the pattern, \10, \11, etc. refer back to substrings if there have been at least that many left parens before the backreference. Otherwise (for backward compatibilty) \10 is the same as \010, a backspace, and \11 the same as \011, a tab. And so on. (\1 through \9 are always backreferences.)

$+ returns whatever the last bracket match matched. `$&` returns the entire matched string. ( $0 used to return the same thing, but not any more.) $` returns everything before the matched string. $' returns everything after the matched string. Examples:

        s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words
        if (/Time: (..):(..):(..)/) {
            $hours = $1;
            $minutes = $2;
            $seconds = $3;
        }

You will note that all backslashed metacharacters in Perl are alphanumeric, such as `\b`, `\w`, `\n`. Unlike some other regular expression languages, there are no backslashed symbols that aren't alphanumeric. So anything that looks like \\, \(, \), <, \>, \{, or \} is always interpreted as a literal character, not a metacharacter. This makes it simple to quote a string that you want to use for a pattern but that you are afraid might contain metacharacters. Simply quote all the non-alphanumeric characters:

        $pattern =~ s/(\W)/\\$1/g;

You can also use the built-in quotemeta() function to do this. An even easier way to quote metacharacters right in the match operator is to say

        /$unquoted\Q$quoted\E$unquoted/

Perl 5 defines a consistent extension syntax for regular expressions. The syntax is a pair of parens with a question mark as the first thing within the parens (this was a syntax error in Perl 4). The character after the question mark gives the function of the extension. Several extensions are already supported:

(?#text)
  A comment. The text is ignored.

(?:regexp)
  This groups things like "()" but doesn't make backrefences like "()" does. So

          split(/\b(?:a|b|c)\b/)

  is like

          split(/\b(a|b|c)\b/)

  but doesn't spit out extra fields.

(?=regexp)
  A zero-width positive lookahead assertion. For example, `/\w+(?=\t)/` matches a word followed by a tab, without including the tab in `$&`.

(?!regexp)
  A zero-width negative lookahead assertion. For example `/foo(?!bar)/` matches any occurrence of "foo" that isn't followed by "bar". Note however that lookahead and lookbehind are NOT the same thing. You cannot use this for lookbehind: `/(?!foo)bar/` will not find an occurrence of "bar" that is preceded by something which is not "foo". That's because the `(?!foo)` is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match. You would have to do something like `/(?foo)...bar/` for that. We say "like" because there's the case of your "bar" not having three characters before it. You could cover that this way: `/(?:(?!foo)...|^..?)bar/`. Sometimes it's still easier just to say:

          if (/foo/ && $` =~ /bar$/)

(?imsx)
  One or more embedded pattern-match modifiers. This is particularly useful for patterns that are specified in a table somewhere, some of which want to be case sensitive, and some of which don't. The case insensitive ones merely need to include `(?i)` at the front of the pattern. For example:

          $pattern = "foobar";
          if ( /$pattern/i )
          # more flexible:
          $pattern = "(?i)foobar";
          if ( /$pattern/ )

The specific choice of question mark for this and the new minimal.matching construct was because 1) question mark is pretty rare in older regular expressions, and 2) whenever you see one, you should stop and "question" exactly what is going on. That's psychology...

## Version 8 Regular Expressions

In case you're not familiar with the "regular" Version 8 regexp routines, here are the pattern-matching rules not described above.

Any single character matches itself, unless it is a metacharacter with a special meaning described here or above. You can cause characters which normally function as metacharacters to be interpreted literally by prefixing them with a "\" (e.g. "\." matches a ".", not any character; "\\" matches a "\"). A series of characters matches that series of characters in the target string, so the pattern `blurfl` would match "blurfl" in the target string.

You can specify a character class, by enclosing a list of characters in `[]`, which will match any one of the characters in the list. If the first character after the "[" is "^", the class matches any character not in the list. Within a list, the "-" character is used to specify a range, so that `a-z` represents all the characters between "a" and "z", inclusive.

Characters may be specified using a metacharacter syntax much like that used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form feed, etc. More generally, \nnn, where nnn is a string of octal digits, matches the character whose ASCII value is nnn. Similarly, \xnn, where nn are hexidecimal digits, matches the character whose ASCII value is nn. The expression \cx matches the ASCII character control-x. Finally, the "." metacharacter matches any character except "\n" (unless you use `/s`).

You can specify a series of alternatives for a pattern using "|" to separate them, so that `fee|fie|foe` will match any of "fee", "fie", or "foe" in the target string (as would `f(e|i|o)e`). Note that the first alternative includes everything from the last pattern delimiter ("(", "[", or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next pattern delimiter. For this reason, it's common practice to include alternatives in parentheses, to minimize confusion about where they start and end. Note however that "|" is interpreted as a literal with square brackets, so if you write `[fee|fie|foe]` you're really only matching `[feio|]`.

Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the nth subpattern later in the pattern using the metacharacter \n. Subpatterns are numbered based on the left to right order of their opening parenthesis. Note that a backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, `(0|0x)\d*\s\1\d*` will match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1 actually matched "0x", even though the rule `0|0x` could potentially match the leading 0 in the second number.