in reply to Re^8: Reusing a complex regexp in multiple spots, escaping the regexp
in thread Reusing a complex regexp in multiple spots, escaping the regexp

> Pretty sure they're not kept as nested objects.

That was my first intuition, and looks like we were right, the final program is the same for the flattened string. 

$ perl -Mre=debug -E'$x=qr/[X0-9a-f]/i; $hr=qr/$x+ - $x+/x;'
Compiling REx "[X0-9a-f]"
Final program:
   1: ANYOF[0-9A-FXa-fx] (11)
  11: END (0)
stclass ANYOF[0-9A-FXa-fx] minlen 1             
Compiling REx "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Final program:                                  
   1: PLUS (12)
   2:   ANYOF[0-9A-FXa-fx] (0)
  12: EXACT <-> (14)                           
  14: PLUS (25)                                
  15:   ANYOF[0-9A-FXa-fx] (0)
  25: END (0)
floating "-" at 1..9223372036854775807 (checking floating) stclass ANY
+OF[0-9A-FXa-fx] plus minlen 3
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Freeing REx: "[X0-9a-f]"
$

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$ perl -Mre=debug -E'$hr=qr/(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+/x;' 
+                        
Compiling REx "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"                 
+                          
Final program:
   1: PLUS (12)
   2:   ANYOF[0-9A-FXa-fx] (0)                  
  12: EXACT <-> (14)                            
  14: PLUS (25)
  15:   ANYOF[0-9A-FXa-fx] (0)                 
  25: END (0)
floating "-" at 1..9223372036854775807 (checking floating) stclass ANY
+OF[0-9A-FXa-fx] plus minlen 3
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
$

[download]

> the regexp would need to be compiled each time around the while loop

I think that's what /o was invented for

Update

So... Consequently any performance penalty coming with qr// could be mitigated by flattening it first and using the resulting string with /o modifier. 🤔

Cheers Rolf
(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery

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Re^10: Reusing a complex regexp in multiple spots, escaping the regexp
by tobyink (Canon) on Apr 16, 2026 at 15:05 UTC

    I think that's what /o was invented for

    If a regexp contains something interpolated, I don't know if /o can do much for it. Consider:

    my $foo = qr/foo/;
while ( get_line($fh) =~ m/($foo)(bar)/ ) {
   ...;
   $foo = qr/FOO/;  # $foo gets changed
}

    [download]

    Even if $foo doesn't change, the fact that it has the potential to change makes any attempt at avoiding recompiling the larger regexp broken.

    perldoc perlre does note that /o is pretty broken.

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      You are showing a semantic where /o MUST NOT be applied. Obviously you shouldn't inhibit recompilation if you need recompilation.

      One of the links sleet listed upthread (or the docs🤔) were explicit that /o would inhibit any recompilation of the regex even with nested variables, hence solving the performance issue in a loop.

      > perldoc perlre does note that /o is pretty broken

      Dunno, a literal quote would be helpful.

      A concrete test - probably with re debug - even more. :)

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      see Wikisyntax for the Monastery

[reply]
[d/l]

        Dunno, a literal quote would be helpful.

        Substitution-specific modifiers described in "s/PATTERN/REPLACEMENT/ms
+ixpodualngcer" in
perlop are:

    e  - evaluate the right-hand side as an expression
    ee - evaluate the right side as a string then eval the result
    o  - pretend to optimize your code, but actually introduce bugs
    r  - perform non-destructive substitution and return the new value

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