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Want to see how a regular expression is compiled? Perl can tell you!

    Debugging regular expressions

       There are two ways to enable debugging output for regular
       expressions.
 
       If your perl is compiled with -DDEBUGGING, you may use the
       -Dr flag on the command line.
 
       Otherwise, one can use re 'debug', which has effects both
       at compile time, and at run time (and is not lexically
       scoped).

       [snip; read the details in the perldebug documentation.]
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The really nice part is that, unlike most of the low-level debugging features, you don't need a DEBUGGING build of Perl to debug your regular expressions.

#!perl -c
use re 'debug';

/at/||/bt/||/ct/||/dt/||/et/||/ft/||/ght/;
/at|bt|ct|dt|et|ft|ght/;
/[abcdef]t|ght/;
/(?:[abcdef]|gh)t/;
/(?:a|b|c|d|e|f|gt)t/;

__END__
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Here's the output from 5.005_03, reformatted for a little extra clarity:

/at/||/bt/||/ct/||/dt/||/et/||/ft/||/ght/;

compiling RE `at'
size 3 first at 1
   1: EXACT <at>(3)
   3: END(0)
anchored `at' at 0 (checking anchored isall) minlen 2

compiling RE `bt'
size 3 first at 1
   1: EXACT <bt>(3)
   3: END(0)
anchored `bt' at 0 (checking anchored isall) minlen 2

compiling RE `ct'
size 3 first at 1
   1: EXACT <ct>(3)
   3: END(0)
anchored `ct' at 0 (checking anchored isall) minlen 2

compiling RE `dt'
size 3 first at 1
   1: EXACT <dt>(3)
   3: END(0)
anchored `dt' at 0 (checking anchored isall) minlen 2

compiling RE `et'
size 3 first at 1
   1: EXACT <et>(3)
   3: END(0)
anchored `et' at 0 (checking anchored isall) minlen 2

compiling RE `ft'
size 3 first at 1
   1: EXACT <ft>(3)
   3: END(0)
anchored `ft' at 0 (checking anchored isall) minlen 2

compiling RE `ght'
size 4 first at 1
   1: EXACT <ght>(4)
   4: END(0)
anchored `ght' at 0 (checking anchored isall) minlen 3


/at|bt|ct|dt|et|ft|ght/;

compiling RE `at|bt|ct|dt|et|ft|ght'
size 23
   1: BRANCH(4)
   2:   EXACT <at>(23)
   4: BRANCH(7)
   5:   EXACT <bt>(23)
   7: BRANCH(10)
   8:   EXACT <ct>(23)
  10: BRANCH(13)
  11:   EXACT <dt>(23)
  13: BRANCH(16)
  14:   EXACT <et>(23)
  16: BRANCH(19)
  17:   EXACT <ft>(23)
  19: BRANCH(23)
  20:   EXACT <ght>(23)
  23: END(0)
minlen 2


/[abcdef]t|ght/;

compiling RE `[abcdef]t|ght'
size 18
   1: BRANCH(14)
   2:   ANYOF(12)
  12:   EXACT <t>(18)
  14: BRANCH(18)
  15:   EXACT <ght>(18)
  18: END(0)
minlen 2


/(?:[abcdef]|gh)t/;

compiling RE `(?:[abcdef]|gh)t'
size 18
   1: BRANCH(12)
   2:   ANYOF(16)
  12: BRANCH(15)
  13:   EXACT <gh>(16)
  15: TAIL(16)
  16: EXACT <t>(18)
  18: END(0)
minlen 2


/(?:a|b|c|d|e|f|gt)t/;

compiling RE `(?:a|b|c|d|e|f|gt)t'
size 25
   1: BRANCH(4)
   2:   EXACT <a>(23)
   4: BRANCH(7)
   5:   EXACT <b>(23)
   7: BRANCH(10)
   8:   EXACT <c>(23)
  10: BRANCH(13)
  11:   EXACT <d>(23)
  13: BRANCH(16)
  14:   EXACT <e>(23)
  16: BRANCH(19)
  17:   EXACT <f>(23)
  19: BRANCH(22)
  20:   EXACT <gt>(23)
  22: TAIL(23)
  23: EXACT <t>(25)
  25: END(0)
minlen 2
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Of course, that's just how the regular expressions are compiled; to see how the regexes are matched -- which is where the optimizer really comes into play -- give the regexes something to match against and watch the output at runtime!

In reply to Re: Regexp Speed Concerns by chipmunk
in thread Regexp Speed Concerns by tadman

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