Programming Perl, 4th Edition

Pattern Modifiers

We’ll discuss the individual pattern-matching operators in a moment, but first we’d like to mention another thing they all have in common, modifiers.

Immediately following the final delimiter of an m//, s///, qr//, y///, or tr/// operator, you may optionally place one or more single-letter modifiers, in any order. For clarity, modifiers are usually written as “the /i modifier” and pronounced “the slash eye modifier”, even though the final delimiter might be something other than a slash. (Sometimes people say “flag” or “option” to mean “modifier”; that’s okay, too.)

Some modifiers change the behavior of the individual operator, so we’ll describe those in detail later. Others change how the regex is interpreted, so we’ll talk about them here. The m//, s///, and qr// operators^[89] all accept the following modifiers after their final delimiter; see Table 5-1.

Table 5-1. Regular expression modifiers

Modifier	Meaning
/i	Ignore alphabetic case distinctions (case-insensitive).
/s	Let . also match newline.
/m	Let ^ and $ also match next to embedded \n.
/x	Ignore (most) whitespace and permit comments in pattern.
/o	Compile pattern once only.
/p	Preserve ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} variables.
/d	Dual ASCII–Unicode mode charset behavior (old default).
/a	ASCII charset behavior.
/u	Unicode charset behavior (new default).
/l	The runtime locale’s charset behavior (default under use locale).

The /i modifier says to match a character in any possible case variation; that is, to match case-insensitively, ...