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POregex to test for correct use of commas
 

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    <p>I'm looping through thousands of strings with various regexes to check for simple errors. I would like to add a regex to check for the correct use of commas.</p> <p>If a comma exists in one of my strings, then it MUST be followed by either whitespace or exactly three digits:</p> <ul> <li>valid: ,\s</li> <li>valid: ,\d\d\d</li> </ul> <p>But if a comma is followed by any other pattern, then it is an error:</p> <ul> <li>invalid: ,\D</li> <li>invalid: ,\d</li> <li>invalid: ,\d\d</li> <li>invalid: ,\d\d\d\d</li> </ul> <p>The best regex I've come up with thus far is:</p> <p><code> Regex CommaError = new Regex(@",(^(\d\d\d)|\S)"); // fails case #2 </code></p> <p>To test, I am using:</p> <pre><code>if (CommaError.IsMatch(", ")) // should NOT match Console.WriteLine("failed case #1"); if (CommaError.IsMatch(",234")) // should NOT match Console.WriteLine("failed case #2"); if (!CommaError.IsMatch("0,a")) // should match Console.WriteLine("failed case #3"); if (!CommaError.IsMatch("0,0")) // should match Console.WriteLine("failed case #4"); if (!CommaError.IsMatch("0,0a1")) // should match Console.WriteLine("failed case #5"); </code></pre> <p>But the regex I gave above fails case #2 (it matches when it should not).</p> <p>I've invested several hours investigating this, and searched the Web for similar regexes, but have hit a brick wall. What's wrong with my regex?</p> <p>Update: Peter posted a comment with a regex that works the way I want:</p> <pre><code>Regex CommaError = new Regex(@",(?!\d\d\d|\s)"); </code></pre> <p>Edit: Well, almost. It fails in this case:</p> <pre><code>if (!CommaError.IsMatch("1,2345")) // should match Console.WriteLine("failed case #6"); </code></pre>
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    1. USgary
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    2. VO
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    1. COWhat language is this for? Different languages use different regex variants which support different features.
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    2. COPeter, I'm using C#.
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    3. CORelated question: Regex: Matching by exclusion, without look-ahead - is it possible? http://stackoverflow.com/questions/466053/regex-matching-by-exclusion-without-look-ahead-is-it-possible
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