Perl’s regular expressions have a simple rule for capturing groups. It counts the order of left parentheses to assign capture variables. Not all capture groups must actually match parts of the string, and Perl doesn’t care if they do. Perl assigns capture groups inside an alternation consecutively, even though it knows that only one branch [...]
In Item 33: “Watch out for match variables, you found out that the match variable $`, $&, and $` come with a performance hit. With all of the module code that you might use, you might be using those variables even though you didn’t code with them yourself. There’s a module that can tell you [...]
Perl keeps track of the last position in a string where it had a successful global match (using the /g flag). You can access this position with the pos operator. With Perl 5.10, you can use the /p switch to get the per-match variable ${^MATCH} instead of the performance-dampening $&: use 5.010; my $string = [...]
The match and substitution operators, as well as regex quoting with qr//, use flags to signal certain behavior of the match or interpretation of the pattern. The flags that change the interpretation of the pattern are listed in the documentation for qr// in perlop (and maybe in other places in earlier versions of the documentation): [...]
