Programming Perl, 4th Edition

The m// Operator (Matching)

m/PATTERN/modifiers
  /PATTERN/modifiers
  ?PATTERN?modifiers (deprecated)

 EXPR =~ m/PATTERN/modifiers
 EXPR =~  /PATTERN/modifiers
 EXPR =~  ?PATTERN?modifiers (deprecated)

The m// operator searches the string in the scalar EXPR for PATTERN. If / or ? is the delimiter, the initial m is optional. Both ? and ' have special meanings as delimiters: the first is a once-only match; the second suppresses variable interpolation and the seven translation escapes (\U and company, described later).

If PATTERN evaluates to a null string, either because you specified it that way using // or because an interpolated variable evaluated to the empty string, the last successfully executed regular expression not hidden within an inner block (or within a split, grep, or map) is used instead.

In scalar context, the operator returns true (1) if successful, false ("") otherwise. This form is usually seen in Boolean context:

if ($shire =~ m/Baggins/) { ... }  # search for Baggins in $shire
if ($shire =~ /Baggins/)  { ... }  # search for Baggins in $shire

if ( m#Baggins# )         { ... }  # search right here in $_
if ( /Baggins/ )          { ... }  # search right here in $_

Used in list context, m// returns a list of substrings matched by the capturing parentheses in the pattern (that is, $1, $2, $3, and so on), as described later under Grouping and Capturing. The numbered variables are still set even when the list is returned. If the match fails in list context, a null list is returned. If the match succeeds ...