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POGreedy vs. Reluctant vs. Possessive Quantifiers
 

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    <p>I found this <a href="http://download.oracle.com/javase/tutorial/essential/regex/quant.html" rel="noreferrer">excellent tutorial</a> on regular expressions and while I intuitively understand what "greedy", "reluctant" and "possessive" quantifiers do, there seems to be a serious hole in my understanding.</p> <p>Specifically, in the following example:</p> <pre><code>Enter your regex: .*foo // greedy quantifier Enter input string to search: xfooxxxxxxfoo I found the text "xfooxxxxxxfoo" starting at index 0 and ending at index 13. Enter your regex: .*?foo // reluctant quantifier Enter input string to search: xfooxxxxxxfoo I found the text "xfoo" starting at index 0 and ending at index 4. I found the text "xxxxxxfoo" starting at index 4 and ending at index 13. Enter your regex: .*+foo // possessive quantifier Enter input string to search: xfooxxxxxxfoo No match found. </code></pre> <p>The explanation mentions <strong>eating</strong> the entire input string, letters been <strong>consumed</strong>, matcher <strong>backing off</strong>, rightmost occurrence of "foo" has been <strong>regurgitated</strong>, etc.</p> <p>Unfortunately, despite the nice metaphors, I still don't understand what is eaten by whom... Do you know of another tutorial that explains (concisely) <em>how</em> regular expressions engines work?</p> <p>Alternatively, if someone can explain in somewhat different phrasing the following paragraph, that would be much appreciated:</p> <blockquote> <p>The first example uses the greedy quantifier .* to find "anything", zero or more times, followed by the letters "f" "o" "o". Because the quantifier is greedy, the .* portion of the expression first eats the entire input string. At this point, the overall expression cannot succeed, because the last three letters ("f" "o" "o") have already been consumed (<strong>by whom?</strong>). So the matcher slowly backs off (<strong>from right-to-left?</strong>) one letter at a time until the rightmost occurrence of "foo" has been regurgitated (<strong>what does this mean?</strong>), at which point the match succeeds and the search ends.</p> <p>The second example, however, is reluctant, so it starts by first consuming (<strong>by whom?</strong>) "nothing". Because "foo" doesn't appear at the beginning of the string, it's forced to swallow (<strong>who</strong> swallows?) the first letter (an "x"), which triggers the first match at 0 and 4. Our test harness continues the process until the input string is exhausted. It finds another match at 4 and 13.</p> <p>The third example fails to find a match because the quantifier is possessive. In this case, the entire input string is consumed by .*+, (<strong>how?</strong>) leaving nothing left over to satisfy the "foo" at the end of the expression. Use a possessive quantifier for situations where you want to seize all of something without ever backing off (<strong>what does back off mean?</strong>); it will outperform the equivalent greedy quantifier in cases where the match is not immediately found.</p> </blockquote>
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    1. CO*Maximal* quantifiers like `*`, `+`, and `?` are ***greedy.*** *Minimal* quantifiers like `*?`, `+?`, and `??` are ***lazy.*** *Possessive* quantifiers like `*+`, `++`, and `?+` are ***sticky.***
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