How can I have optional matches in a Perl regex?

Question

I have a string I read from a configuration file. Structure of the string is as follows;

(long_string)long_string(long_string)

Any item in brackets, including the brackets themselves, are optional. I have the following regular expression matching the whole string but I could not figure out how to make some parts of the regular expression optional with "?".

Here are a few valid strings for input

(a)like(1)
like(very long string here)
like

Here is my regexp only matching the first one;

^\((?<short>.*)\)(?<text>.*)\((?<return>.*)\)$

How can I convert my regexp to make brackets optional for a match?

Answer 1

Surround the two sub-patterns with non-matching groups (?:expr) and make them optional:

^(?:\((?<short>.*)\))?(?<text>.*)(?:\((?<return>.*)\))?$

And if possible make the universal expression .* more specific, maybe with [^()]+:

^(?:\((?<short>[^()]+)\))?(?<text>[^()]+)(?:\((?<return>[^()]+)\))?$

Answer 2

What I would do is wrap the ( and ) with your grouping members, so instead of

\((?<short>.*)\)

change it to:

(\(<short>.*\))

That way it will match the ()'s along with the inner text. Then, if they are present use another regular expression to eliminate the parentheses.

I'm not very familiar with the named matches syntax so the group syntax might be off but you should get the idea.

Answer 3

Give this a try...

string[] strings = new string[] { "(a)like(1)", "like(very long string here)", "like" };
foreach (string s in strings)
{
    System.Text.RegularExpressions.Match match = System.Text.RegularExpressions.Regex.Match(s, @"^(\((?<short>.)\))?(?<text>.+)?(\((?<return>.+)\))?$");
    if (match.Success)
    {
        // do logic to handle the match
    }
}

Answer 4

Well, just make them optional, then:

^(?<short>\(.*\))?(?<text>.*)(?<return>\(.*\))?$

I'm no big fan of named captures, they tend to make it look more complicated than it is (at least for me). Also, I recommend against using ".*". My suggestion:

^(\([^)]*\))?([^(]*)(\([^)]*\))?$

and go for match group 2. But if you insist on using named captures:

^(?<short>\([^)]*\))?(?<text>[^(]*)(?<return>\([^)]*\))?$

Answer 5

Using the code below, you will always get a @matches array consisting of three elements. If one of the optional parts did not match, the corresponding entry will be undef.

#!/usr/bin/perl

use strict;
use warnings;

my $optional = qr/(?:\(([^)]+?)\))?/;
my $required = qr/([^()]+)/;

while ( my $line = <DATA> ) {
    chomp $line;
    last unless $line =~ /\S/;

    if ( my @matches = ($line =~ /$optional$required$optional/) ) {
        no warnings 'uninitialized';
        print "---$_---\n" for @matches;
    }
}

__DATA__
(a)like(1)
like(very long string here)
like