How do you debug a regex? [closed]

You buy RegexBuddy and use its built in debug feature. If you work with regexes more than twice a year, you will make this money back in time saved in no time. RegexBuddy will also help you to create simple and complex regular expressions, and even generate the code for you in a variety of languages.

Also, according to the developer, this tool runs nearly flawlessly on Linux when used with WINE.

With Perl 5.10, use re 'debug';. (Or debugcolor, but I can't format the output properly on Stack Overflow.)

$ perl -Mre=debug -e'"foobar"=~/(.)\1/'
Compiling REx "(.)\1"
Final program:
   1: OPEN1 (3)
   3:   REG_ANY (4)
   4: CLOSE1 (6)
   6: REF1 (8)
   8: END (0)
minlen 1
Matching REx "(.)\1" against "foobar"
   0 <> <foobar>             |  1:OPEN1(3)
   0 <> <foobar>             |  3:REG_ANY(4)
   1 <f> <oobar>             |  4:CLOSE1(6)
   1 <f> <oobar>             |  6:REF1(8)
                                  failed...
   1 <f> <oobar>             |  1:OPEN1(3)
   1 <f> <oobar>             |  3:REG_ANY(4)
   2 <fo> <obar>             |  4:CLOSE1(6)
   2 <fo> <obar>             |  6:REF1(8)
   3 <foo> <bar>             |  8:END(0)
Match successful!
Freeing REx: "(.)\1"

Also, you can add whitespace and comments to regexes to make them more readable. In Perl, this is done with the /x modifier. With pcre, there is the PCRE_EXTENDED flag.

"foobar" =~ /
    (.)  # any character, followed by a
    \1   # repeat of previously matched character
/x;

pcre *pat = pcre_compile("(.)  # any character, followed by a\n"
                         "\\1  # repeat of previously matched character\n",
                         PCRE_EXTENDED,
                         ...);
pcre_exec(pat, NULL, "foobar", ...);