Programming Perl, 4th Edition

Binding Operators

Binary =~ binds a string expression to a pattern match, substitution, or transliteration (loosely called translation). These operations would otherwise search or modify the string contained in $_ (the default variable). The string you want to bind is put on the left, while the operator itself is put on the right. The return value in scalar context generally indicates the success or failure of the operator on the right, since the binding operator doesn’t really do anything on its own. The exception to this is when using the /r modifier with substitution (s///) or transliteration (y///, tr///), which returns a copy of the modified string. Behavior in list context depends on the particular operator.

If the right argument is an expression rather than a pattern match, substitution, or transliteration, it will be interpreted as a search pattern at runtime. That is to say, $_ =~ $pat is equivalent to $_ =~ /$pat/. This is less efficient than an explicit search, since the pattern must be checked and possibly recompiled every time the expression is evaluated. You can avoid this recompilation by precompiling the original pattern using the qr// (quote regex) operator.

Binary !~ is just like =~ except the return value is negated logically. Binary “!~” that attempts to use the /r modifier for a nondestructive substitution or transliteration is a syntax error. Apart from that, the following expressions are functionally equivalent:

$string !~ /pattern/ !( $string =~ /pattern/ ) ...