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copied!<p>In your first expression the lookahead assertion is matching your input as (<code>greedy</code>). </p> <p>Running a debug test on your regular expression displays the following.</p> <pre><code>Matching REx "(?=.*\d)\w{4,}" against "!abcae20" 0 <> <!abcae20> | 1: IFMATCH[0](8) 0 <> <!abcae20> | 3: STAR(5) REG_ANY can match 8 times out of 2147483647... 8 <!abcae20> <> | 5: DIGIT(6) failed... 7 <!abcae2> <0> | 5: DIGIT(6) 8 <!abcae20> <> | 6: SUCCEED(0) subpattern success... 0 <> <!abcae20> | 8: CURLY {4,32767}(11) ALNUMU can match 0 times out of 2147483647... failed... Match failed </code></pre> <ul> <li><p>Explanation on what caused the match to fail..</p> <ol> <li><p><<strong>!abcae20</strong>></p> <pre><code>The greedy quantifier first matches as much as possible. So the .* here is matching the entire string. </code></pre></li> <li><p><<code>!abcae20</code>></p> <pre><code>Then tries to match any numeric character following, but there are no characters left to match. </code></pre></li> <li><p><<code>!abcae2</code><strong>0</strong>></p> <pre><code>So it backtracks making the greedy match, match one less character leaving the --> 0 <-- at the end unmatched. </code></pre></li> <li><p><<code>!abcae20</code>></p> <pre><code>So it backtracks again matching one less leaving it unmatched. </code></pre></li> <li><p><<strong>!abcae20</strong>></p> <pre><code>So it backtracks one more step matching one less again and failing your match. </code></pre></li> </ol></li> <li><p>Regular expression explained:</p> <pre><code>(?= look ahead to see if there is: .* any character except \n (0 or more times) \d digits (0-9) ) end of look-ahead \w{4,} word characters (a-z, A-Z, 0-9, _) (at least 4 times) </code></pre></li> </ul> <p>Your second expression does match <code>!</code> with the preceding non-greedy <code>.*?</code> followed by your lookahead assertion which matches <code>!abcae2</code>, then backtracks to match your word characters and complete string.</p> <ul> <li><p>Regular expression explained:</p> <pre><code>.*? any character except \n (0 or more times) (?= look ahead to see if there is: .* any character except \n (0 or more times) \d digits (0-9) ) end of look-ahead \w{4,} word characters (a-z, A-Z, 0-9, _) (at least 4 times) </code></pre></li> </ul>

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