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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 from Henry Spencer's freely redistributable reimplementation of the V8 routines.) In addition, `\w' matches an alphanumeric character (including `_') and `\W' a nonalphanumeric. Word boundaries may be matched by `\b', and non-boundaries by `\B'. A whitespace character is matched by `\s', non-whitespace by `\S'. A numeric character is matched by `\d', non-numeric by `\D'. You may use `\w', `\s' and `\d' within character classes. Also, `\n', `\r', `\f', `\t' and `\NNN' have their normal interpretations. Within character classes `\b' represents backspace rather than a word boundary. Alternatives may be separated by `|'. The bracketing construct `(...)' may also be used, in which case `\<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. 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;

By default, the `^' character is only guaranteed to match at the beginning of the string, the `$' character only at the end (or before the newline at the end) and perl does certain optimizations with the assumption that the string contains only one line. The behavior of `^' and `$' on embedded newlines will be inconsistent. 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 `$' will match before any newline. At the cost of a little more overhead, you can do this by setting the variable `$*' to 1. Setting it back to 0 makes perl revert to its old behavior.

To facilitate multi-line substitutions, the `.' character never matches a newline (even when `$*' is 0). In particular, the following leaves a newline on the `$_' string:

$_ = <STDIN>;

If the newline is unwanted, try one of

chop; s/.*(some_string).*/$1/;
/(some_string)/ && ($_ = $1);

Any item of a regular expression may be followed with digits in curly brackets of the form `{n,m}', where n gives the minimum number of times to match the item and m gives the maximum. The form `{n}' is equivalent to `{n,n}' and matches exactly n times. The form `{n,}' matches n or more 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.

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;

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