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    copied!<p>What you are proposing is difficult but <a href="https://stackoverflow.com/questions/2348694/how-do-you-debug-a-regex/2348711#2348711">doable</a>. </p> <p>If I can paraphrase what I understand, you are wanting to find out how far a failing match got into a match. In order to do this, you need to be able to parse a regex. </p> <p>The best regex parser is probably to use Perl itself with the <code>-re=debug</code> command line switch:</p> <pre><code>$ perl -Mre=debug -e'"abcdefghijklmnopqr"=~/gh[ijkl]{5}/' Compiling REx "gh[ijkl]{5}" Final program: 1: EXACT &lt;gh&gt; (3) 3: CURLY {5,5} (16) 5: ANYOF[i-l][] (0) 16: END (0) anchored "gh" at 0 (checking anchored) minlen 7 Guessing start of match in sv for REx "gh[ijkl]{5}" against "abcdefghijklmnopqr" Found anchored substr "gh" at offset 6... Starting position does not contradict /^/m... Guessed: match at offset 6 Matching REx "gh[ijkl]{5}" against "ghijklmnopqr" 6 &lt;bcdef&gt; &lt;ghijklmnop&gt; | 1:EXACT &lt;gh&gt;(3) 8 &lt;defgh&gt; &lt;ijklmnopqr&gt; | 3:CURLY {5,5}(16) ANYOF[i-l][] can match 4 times out of 5... failed... Match failed Freeing REx: "gh[ijkl]{5}" </code></pre> <p>You can shell out that Perl command line with your regex and parse the return of stdout. Look for the `</p> <p>Here is a matching regex:</p> <pre><code>$ perl -Mre=debug -e'"abcdefghijklmnopqr"=~/gh[ijkl]{3}/' Compiling REx "gh[ijkl]{3}" Final program: 1: EXACT &lt;gh&gt; (3) 3: CURLY {3,3} (16) 5: ANYOF[i-l][] (0) 16: END (0) anchored "gh" at 0 (checking anchored) minlen 5 Guessing start of match in sv for REx "gh[ijkl]{3}" against "abcdefghijklmnopqr" Found anchored substr "gh" at offset 6... Starting position does not contradict /^/m... Guessed: match at offset 6 Matching REx "gh[ijkl]{3}" against "ghijklmnopqr" 6 &lt;bcdef&gt; &lt;ghijklmnop&gt; | 1:EXACT &lt;gh&gt;(3) 8 &lt;defgh&gt; &lt;ijklmnopqr&gt; | 3:CURLY {3,3}(16) ANYOF[i-l][] can match 3 times out of 3... 11 &lt;ghijk&gt; &lt;lmnopqr&gt; | 16: END(0) Match successful! Freeing REx: "gh[ijkl]{3}" </code></pre> <p>You will need to build a parser that can handle <a href="http://perl.active-venture.com/pod/perldebguts-debugregexp.html" rel="nofollow noreferrer">the return</a> from the Perl re debugger. The left hand and right hand angle braces show the distance into the string as the regex engine is trying to match.</p> <p>This is not an easy project btw...</p>
 

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