This should probably get you started. 
use strict;

while (<DATA>) {
    #OLDmy ($sign, $num, $den) = m@(-)?(\d+)/?(\d*)@;
    my ($sign, $num, $den) = m@(-)?(\d+)(?:/(\d+))?@;
    print "sign:$sign numerator:$num denominator:$den\n";
}

__DATA__
23
-35
-23/43
2/3
5/

[download]

-jbWare

Update:Fixed regex based on ikegami's suggestion. Also pulled / out of the denominator in ikegami's regex.

In reply to Re: regex by jbware
in thread How do I detect if a number has a minus sign? by algonquin

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