#! /usr/local/bin/perl -w
use Regexp::Assemble;
my $ra = Regexp::Assemble->new->add(
    'Error 123 on SystemA file not found error', 
    'Error 123 on SystemB file not found error', 
    'Error 123 on SystemC file not found error'
);
print $ra->re;

##</code><code>##

(?-xism:Error 123 on System[ABC] file not found error)

##</code><code>##

#! /usr/local/bin/perl -w
use Regexp::Assemble;
my $ra = Regexp::Assemble->new->add(
    'Error 124 on User1:FileA no space left', 
    'Error 124 on User2:FileB no space left', 
    'Error 124 on User3:FileC no space left'
);
print $ra->re;

##</code><code>##

(?-xism:Error 124 on User(?:1:FileA|2:FileB|3:FileC) no space left)