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    <p>I haven't found any nice debugging utility for PCRE regexps. The more I could find was how to dump the bytecode:</p> <pre><code>$ pcretest -b PCRE version 7.6 2008-01-28 re&gt; /^((.)(?1)\2|.?)$/x ------------------------------------------------------------------ 0 39 Bra 3 ^ 4 26 CBra 1 9 6 CBra 2 14 Any 15 6 Ket 18 6 Once 21 4 Recurse 24 6 Ket 27 \2 30 5 Alt 33 Any? 35 31 Ket 38 $ 39 39 Ket 42 End ------------------------------------------------------------------ </code></pre> <p>Perl has better debugging tools than PCRE, try <code>echo 123454321 | perl -Mre=debug -ne '/^((.)(?1)\2|.?)$/x'</code>. This gives not only some bytecode that is similar to PCRE's one, but it also shows each step, and the consumed and remaining parts of the input at each step:</p> <pre><code>Compiling REx "^((.)(?1)\2|.?)$" Final program: 1: BOL (2) 2: OPEN1 (4) 4: BRANCH (15) 5: OPEN2 (7) 7: REG_ANY (8) 8: CLOSE2 (10) 10: GOSUB1[-8] (13) 13: REF2 (19) 15: BRANCH (FAIL) 16: CURLY {0,1} (19) 18: REG_ANY (0) 19: CLOSE1 (21) 21: EOL (22) 22: END (0) floating ""$ at 0..2147483647 (checking floating) anchored(BOL) minlen 0 Guessing start of match in sv for REx "^((.)(?1)\2|.?)$" against "12321" Found floating substr ""$ at offset 5... Guessed: match at offset 0 Matching REx "^((.)(?1)\2|.?)$" against "12321" 0 &lt;&gt; &lt;12321&gt; | 1:BOL(2) 0 &lt;&gt; &lt;12321&gt; | 2:OPEN1(4) 0 &lt;&gt; &lt;12321&gt; | 4:BRANCH(15) 0 &lt;&gt; &lt;12321&gt; | 5: OPEN2(7) 0 &lt;&gt; &lt;12321&gt; | 7: REG_ANY(8) 1 &lt;1&gt; &lt;2321&gt; | 8: CLOSE2(10) 1 &lt;1&gt; &lt;2321&gt; | 10: GOSUB1[-8](13) 1 &lt;1&gt; &lt;2321&gt; | 2: OPEN1(4) 1 &lt;1&gt; &lt;2321&gt; | 4: BRANCH(15) 1 &lt;1&gt; &lt;2321&gt; | 5: OPEN2(7) 1 &lt;1&gt; &lt;2321&gt; | 7: REG_ANY(8) 2 &lt;12&gt; &lt;321&gt; | 8: CLOSE2(10) 2 &lt;12&gt; &lt;321&gt; | 10: GOSUB1[-8](13) 2 &lt;12&gt; &lt;321&gt; | 2: OPEN1(4) 2 &lt;12&gt; &lt;321&gt; | 4: BRANCH(15) 2 &lt;12&gt; &lt;321&gt; | 5: OPEN2(7) 2 &lt;12&gt; &lt;321&gt; | 7: REG_ANY(8) 3 &lt;123&gt; &lt;21&gt; | 8: CLOSE2(10) 3 &lt;123&gt; &lt;21&gt; | 10: GOSUB1[-8](13) 3 &lt;123&gt; &lt;21&gt; | 2: OPEN1(4) 3 &lt;123&gt; &lt;21&gt; | 4: BRANCH(15) 3 &lt;123&gt; &lt;21&gt; | 5: OPEN2(7) 3 &lt;123&gt; &lt;21&gt; | 7: REG_ANY(8) 4 &lt;1232&gt; &lt;1&gt; | 8: CLOSE2(10) 4 &lt;1232&gt; &lt;1&gt; | 10: GOSUB1[-8](13) 4 &lt;1232&gt; &lt;1&gt; | 2: OPEN1(4) 4 &lt;1232&gt; &lt;1&gt; | 4: BRANCH(15) 4 &lt;1232&gt; &lt;1&gt; | 5: OPEN2(7) 4 &lt;1232&gt; &lt;1&gt; | 7: REG_ANY(8) 5 &lt;12321&gt; &lt;&gt; | 8: CLOSE2(10) 5 &lt;12321&gt; &lt;&gt; | 10: GOSUB1[-8](13) 5 &lt;12321&gt; &lt;&gt; | 2: OPEN1(4) 5 &lt;12321&gt; &lt;&gt; | 4: BRANCH(15) 5 &lt;12321&gt; &lt;&gt; | 5: OPEN2(7) 5 &lt;12321&gt; &lt;&gt; | 7: REG_ANY(8) failed... 5 &lt;12321&gt; &lt;&gt; | 15: BRANCH(19) 5 &lt;12321&gt; &lt;&gt; | 16: CURLY {0,1}(19) REG_ANY can match 0 times out of 1... 5 &lt;12321&gt; &lt;&gt; | 19: CLOSE1(21) EVAL trying tail ... 9d86dd8 5 &lt;12321&gt; &lt;&gt; | 13: REF2(19) failed... failed... BRANCH failed... 4 &lt;1232&gt; &lt;1&gt; | 15: BRANCH(19) 4 &lt;1232&gt; &lt;1&gt; | 16: CURLY {0,1}(19) REG_ANY can match 1 times out of 1... 5 &lt;12321&gt; &lt;&gt; | 19: CLOSE1(21) EVAL trying tail ... 9d86d70 5 &lt;12321&gt; &lt;&gt; | 13: REF2(19) failed... 4 &lt;1232&gt; &lt;1&gt; | 19: CLOSE1(21) EVAL trying tail ... 9d86d70 4 &lt;1232&gt; &lt;1&gt; | 13: REF2(19) failed... failed... BRANCH failed... 3 &lt;123&gt; &lt;21&gt; | 15: BRANCH(19) 3 &lt;123&gt; &lt;21&gt; | 16: CURLY {0,1}(19) REG_ANY can match 1 times out of 1... 4 &lt;1232&gt; &lt;1&gt; | 19: CLOSE1(21) EVAL trying tail ... 9d86d08 4 &lt;1232&gt; &lt;1&gt; | 13: REF2(19) failed... 3 &lt;123&gt; &lt;21&gt; | 19: CLOSE1(21) EVAL trying tail ... 9d86d08 3 &lt;123&gt; &lt;21&gt; | 13: REF2(19) failed... failed... BRANCH failed... 2 &lt;12&gt; &lt;321&gt; | 15: BRANCH(19) 2 &lt;12&gt; &lt;321&gt; | 16: CURLY {0,1}(19) REG_ANY can match 1 times out of 1... 3 &lt;123&gt; &lt;21&gt; | 19: CLOSE1(21) EVAL trying tail ... 9d86ca0 3 &lt;123&gt; &lt;21&gt; | 13: REF2(19) 4 &lt;1232&gt; &lt;1&gt; | 19: CLOSE1(21) EVAL trying tail ... 0 4 &lt;1232&gt; &lt;1&gt; | 13: REF2(19) 5 &lt;12321&gt; &lt;&gt; | 19: CLOSE1(21) 5 &lt;12321&gt; &lt;&gt; | 21: EOL(22) 5 &lt;12321&gt; &lt;&gt; | 22: END(0) Match successful! Freeing REx: "^((.)(?1)\2|.?)$" </code></pre> <p>As you can see, Perl first consumes all input recursing until <code>(.)</code> fails, then starts backtracking and trying the second branch from the alternation <code>.?</code> and the remainder of the first part <code>\2</code>, when that fails it backtracks, until it finally succeeds.</p>
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