Just a thought:

#! /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;
[download]

Produces:

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

And:

#! /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;
[download]

Outputs:

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

See also:

Regexp::Assemble
grinder's scratchpad
Why machine-generated solutions will never cease to amaze me

HTH,