I used to (before use strict) compose regexps by hiding the components inside an if, e.g.: 
$complex_re = /^($ip) ($host) ($msg)$/ if (
    $ip   = /\d+\.\d+\.\d+\.\d+/,
    $host = /[\-\.\w]+/,
    $msg  = /.*/
);

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Nowadays, I use strict and eval, e.g. 
my $complex_re = eval {
    my $ip   = qr/\d+\.\d+\.\d+\.\d+/;
    my $host = qr/[\-\.\w]+/;
    my $msg  = qr/.*/;

    return /^($ip) ($host) ($msg)$/;
);

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We pay about a 5% penalty for the eval when we use this in a tight loop, however we can solve that by moving the regexp creation outside the loop. 
                 Rate    with eval without eval
with eval    116279/s           --          -5%
without eval 121951/s           5%           --

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Patrick

In reply to Regexp Legibility by patrickhaller
in thread Regexp do's and don'ts by muba

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