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POHow can I specify an optional capture group in this RegEx?
 

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  1. POHow can I specify an optional capture group in this RegEx?
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    <p><strong>How can I fix this RegEx to optionally capture a file extension?</strong></p> <p>I am trying to match a string with an optional component, but something appears to be wrong. (The strings being matched are from a printer log.)</p> <p><br>My RegEx (.NET Flavor) is as follows:</p> <pre><code>.*(header_\d{10,11}_).*(_.*_\d{8}).*(\.\w{3,4}).* ------------------------------------------- .* # Ignore some garbage in the front (header_ # Match the start of the file name, \d{10,11}_) # including the ID (10 - 11 digits) .* # Ignore the type code in the middle (_.*_\d{8}) # Match some random characters, then an 8-digit date .* # Ignore anything between this and the file extension (\.\w{3,4}) # Match the file extension, 3 or 4 characters long .* # Ignore the rest of the string </code></pre> <p><br>I expect this to match strings like:</p> <pre><code>str1 = "header_0000000602_t_mc2e1nrobr1a3s55niyrrqvy_20081212[1].doc [Compatibility Mode]" str2 = "Microsoft PowerPoint - header_00000000076_d_al41zguyvgqfj2454jki5l55_20071203[1].txt" str3 = "header_00000000076_d_al41zguyvgqfj2454jki5l55_20071203[1]" </code></pre> <p><br>Where the capture groups return something like:</p> <pre><code>$1 = header_0000000602_ $2 = _mc2e1nrobr1a3s55niyrrqvy_20081212 $3 = .doc </code></pre> <p><br>Where $3 can be empty if no file extension is found. $3 is the optional part, as you can see in str3 above.</p> <p>If I add "?" to the end of the third capture group "(.\w{3,4})?", the RegEx no longer captures $3 for any string. If I add "+" instead "(.\w{3,4})+", the RegEx no longer captures str3 at all, which is to be expected.</p> <p>I feel that using "?" at the end of the third capture group is the appropriate thing to do, but it doesn't work as I expect. I am probably being too naive with the ".*" sections that I use to ignore parts of the string.</p> <p><br>Doesn't Work As Expected:</p> <pre><code>.*(header_\d*_).*(_.*_.{8}).*(\.\w{3,4})?.* </code></pre>
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    1. COwhich regex implementation are you using?
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