in reply to Re^7: 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. I believe when the larger regexp is being compiled, any nested regexps are simply stringified and interpolated into it.

In the original post that could be a problem as the regexp would need to be compiled each time around the while loop. As long as you're compiling it outside the loop, I don't see it being much of an issue.

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Re^9: Reusing a complex regexp in multiple spots, escaping the regexp
by LanX (Saint) on Apr 16, 2026 at 14:39 UTC
    > 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]"
$

    [download]

    $ 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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      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

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