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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
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11.1 Modifiers

Matching operations can have various modifiers. Modifiers that relate to the interpretation of the regular expression inside are listed below. Modifiers that alter the way a regular expression is used by Perl are detailed in 7.30 and 7.32.

Treat string as multiple lines. That is, change "^" and "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string.
Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which normally it would not match. Used together, as /ms, they let the "." match any character whatsoever, while still allowing "^" and "$" to match, respectively, just after and just before newlines within the string.
Do case-insensitive pattern matching. If use locale is in effect, the case map is taken from the current locale. See "Perl locale handling (internationalization and localization)" (perllocale) in the Perl Unicode and Locales Manual.
Extend your pattern's legibility by permitting whitespace and comments.
Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} are available for use after matching.
g and c
Global matching, and keep the Current position after failed matching. Unlike i, m, s and x, these two flags affect the way the regex is used rather than the regex itself. See "Using regular expressions in Perl" (perlretut) in Perl Tutorials for further explanation of the g and c modifiers.

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

The /x modifier itself needs a little more explanation. It tells the regular expression parser to ignore most whitespace that is neither backslashed nor within a character class. You can use this to break up your regular expression into (slightly) more readable parts. The # character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code. This also means that if you want real whitespace or # characters in the pattern (outside a character class, where they are unaffected by /x), then you'll either have to escape them (using backslashes or \Q...\E) or encode them using octal, hex, or \N{} escapes. Taken together, these features go a long way towards making Perl's regular expressions more readable. Note that you have to be careful not to include the pattern delimiter in the comment--perl has no way of knowing you did not intend to close the pattern early. See the C-comment deletion code in 7. Also note that anything inside a \Q...\E stays unaffected by /x. And note that /x doesn't affect whether space interpretation within a single multi-character construct. For example in \x{...}, regardless of the /x modifier, there can be no spaces. Same for a quantifier such as {3} or {5,}. Similarly, (?:...) can't have a space between the ? and :, but can between the ( and ?. Within any delimiters for such a construct, allowed spaces are not affected by /x, and depend on the construct. For example, \x{...} can't have spaces because hexadecimal numbers don't have spaces in them. But, Unicode properties can have spaces, so in \p{...} there can be spaces that follow the Unicode rules, for which see "Properties accessible through \p{} and \P{}" (perluniprops) in the Perl Unicode and Locales Manual.

ISBN 9781906966027Perl Language Reference ManualSee the print edition