Well, if the prefixes never contain spaces, Zaxo's fix works perfectly.

Though, if you cannot make that assumption, but you can make the assumption that the actual chemical names never have a space in them, you can use japhy's sexeger techiniques. That is, reverse the string and apply the regex there.

#!/usr/bin/perl -w
use strict;

my @data = ('12 chem1',
              '1/2 chem2',
              '(N+1) chem3',
              '2M chem4',
              'N chem5');
my $chem;

foreach (@data)
{
    $chem = reverse $_;
    $chem =~ s/^\S+//;
    $chem = reverse $chem;
    print $chem . "\n";
}
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Jeremy