perlrebackslash (1) Linux Manual Page

NAME

perlrebackslash – Perl Regular Expression Backslash Sequences and Escapes

DESCRIPTION

The top level documentation about Perl regular expressions is found in perlre.

This document describes all backslash and escape sequences. After explaining the role of the backslash, it lists all the sequences that have a special meaning in Perl regular expressions (in alphabetical order), then describes each of them.

Most sequences are described in detail in different documents; the primary purpose of this document is to have a quick reference guide describing all backslash and escape sequences.

The backslash

In a regular expression, the backslash can perform one of two tasks: it either takes away the special meaning of the character following it (for instance, "\|" matches a vertical bar, it’s not an alternation), or it is the start of a backslash or escape sequence.

The rules determining what it is are quite simple: if the character following the backslash is an ASCII punctuation (non-word) character (that is, anything that is not a letter, digit, or underscore), then the backslash just takes away any special meaning of the character following it.

If the character following the backslash is an ASCII letter or an ASCII digit, then the sequence may be special; if so, it’s listed below. A few letters have not been used yet, so escaping them with a backslash doesn’t change them to be special. A future version of Perl may assign a special meaning to them, so if you have warnings turned on, Perl issues a warning if you use such a sequence. [1].

It is however guaranteed that backslash or escape sequences never have a punctuation character following the backslash, not now, and not in a future version of Perl 5. So it is 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 the backslash with a backslash: "/\/" matches a single backslash.

[1]

There is one exception. If you use an alphanumeric character as the delimiter of your pattern (which you probably shouldn’t do for readability reasons), 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 []."