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    <p>This is not an anomaly: <code>.*</code> can match anything.</p> <p>You ask to replace all occurrences:</p> <ul> <li>the first occurrence does match the whole string, the regex engine therefore starts from the end of input for the next match;</li> <li>but <code>.*</code> also matches an empty string! It therefore matches an empty string at the end of the input, and replaces it with <code>a</code>.</li> </ul> <p>Using <code>.+</code> instead will not exhibit this problem since this regex cannot match an empty string (it requires at least one character to match).</p> <p>Or, use <code>.replaceFirst()</code> to only replace the first occurrence:</p> <pre><code>"test".replaceFirst(".*", "a") ^^^^^^^^^^^^ </code></pre> <p>Now, why <code>.*</code> behaves like it does and <em>does not match more than twice</em> (it theoretically could) is an interesting thing to consider. See below:</p> <pre><code># Before first run regex: |.* input: |whatever # After first run regex: .*| input: whatever| #before second run regex: |.* input: whatever| #after second run: since .* can match an empty string, it it satisfied... regex: .*| input: whatever| # However, this means the regex engine matched an empty input. # All regex engines, in this situation, will shift # one character further in the input. # So, before third run, the situation is: regex: |.* input: whatever&lt;|ExhaustionOfInput&gt; # Nothing can ever match here: out </code></pre> <p>Note that, as @A.H. notes in the comments, not all regex engines behave this way. GNU <code>sed</code> for instance will consider that it has exhausted the input after the first match.</p>
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    1. COAgreed. This is true for Perl too. `perl -le '$x = "test"; $x =~ s/.*/a/g; print $x'` yields "aa".
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    2. CO@ChrisDolan: `sed` yields only `a`, but I doubt its a bug. :-)
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    3. CO@A.H. yes indeed... I need to read "Mastering Regular Expressions" again
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