So I have a regex in perl which goes like,

my $texttosub = "log10(blackcat)";
#the value of "blackcat" can be found in a hash called "%cats"
while ( $texttosub =~ s|([a-zA-Z][A-Za-z_0-9]+)|$cats{$1}|i ){        
+  
  print ("\n", " The value of cat = ", eval ($texttosub) );
  ..do something..
}
sub log10{....}
[download]

My question is, How do I ignore "log10" and only match "blackcat" for substitution? So that I can evaluate that "$texttosub" line and print of the log10 value of that "blackcat". What I am looking for is, say blackcat=>5, whitecat=>10,orangecat=>20, then, $texttosub = "log10(blackcat)*whitecat*(log10(orangecat))" ===>must become log10(5)*10*(log10(20)). I tried this,

 while ( $texttosub =~ s/(?!log10)|([a-zA-Z][A-Za-z_0-9]+)/$cats{$1}/i
+ ){
   ..do something..
  }
[download]

But I am getting an infinite loop for some reason. Thanks in advance.