Occassionally, you'll find a need to write a complicated regular expression, but you want to be able to group elements of it without capturing them to a dollar/number ($1, $2, etc.) variable. For example, imagine a simple log file in this format: 
line number: action filename

[download]
A typical section of the log may have data as follows: 
9248: OPEN perl.doc
9249: DELETE incriminating_evidence.txt
9250: EDIT autoexec.bat

[download]
Ignoring the over-simplicity of this example, what if you wanted to write a logfile analyzer that justs extracts records that have been deleted or edited? One way, though perhaps not the best way, to do that would be the following: 
while (<>) {
    if ( /^(\d+):\s(?:EDIT|DELETE)\s(.*)$/ ) {
        $results{ $1 } = $2;
    }
}

[download]
What the (?:xxx) does is allow me to group that alternation without capturing the value. It's useful in that it is faster than capturing the value and there's no sense in capturing data if I really don't need it (though I'd probably want to know if a file was edited or deleted).

Also, note that I do have a dot star at the end. This is appropriate in this case because it's doing exactly what I wanted it to do: slurp up the rest of the line.

Also, in case you weren't aware: a regular expression without a binding operator ('=~' or '!~') automatically matches against $_, as in the above example.

Cheers,
Ovid

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In reply to (Ovid non-backreferencing parens) Re(4): Regular Expression Matching by Ovid
in thread Regular Expression Matching by daviddhall

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