Ummmm. It most certainly does :D. There is no variable interpolation going on this Regex. Not the one I put up there anyway. Point it out if you don't mind. - kel -

I don't understand.. I was noting that the /o flag is only useful for compiling a regex once, to avoid recompiling it later. However, this is only a factor if the regex has an interpolated variable. In other words, if Perl notices that the regex will be constant between calls, it will just compile it once anyway. The /o flag does nothing in this case. If Perl sees any interpolated variables, it has to assume that this variable may be different between regex tests, so it recompiles it every time, unless the /o option is given.

Benchmark: timing 10000000 iterations of comp, exist, regex, regexo...
      comp: 53 wallclock secs (53.96 usr +  0.00 sys = 53.96 CPU)
     exist: 49 wallclock secs (49.23 usr +  0.00 sys = 49.23 CPU)
     regex: 78 wallclock secs (77.40 usr +  0.00 sys = 77.40 CPU)
    regexo: 78 wallclock secs (77.12 usr +  0.00 sys = 77.12 CPU)
[download]

As you can see regexo isn't showing much difference. Could this be because I'm using /^[A..Z]$/o instead of your suggested /^[A-Z]$/o, or are they functionally equivalent?

Pete