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POc# regular expression match at specific index in string?
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copied!<p>I'd like to test if a regex will match part of a string at a specific index (and only starting at that specific index). For example, given the string "one two 3 4 five", I'd like to know that, at index 8, the regular expression [0-9]+ will match "3". RegularExpression.IsMatch and Match both take a starting index, however they both will search the entire rest of the string for a match if necessary.</p> <pre><code>string text="one two 3 4 five"; Regex num=new Regex("[0-9]+"); //unfortunately num.IsMatch(text,0) also finds a match and returns true Console.WriteLine("{0} {1}",num.IsMatch(text, 8),num.IsMatch(text,0)); </code></pre> <p>Obviously, I could check if the resulting match starts at the index I am interested in, but I will be doing this a large number of times on large strings, so I don't want to waste time searching for matches later on in the string. Also, I won't know in advance what regular expressions I will actually be testing against the string.</p> <p>I don't want to:</p> <ol> <li>split the string on some boundary like whitespace because in my situation I won't know in advance what a suitable boundary would be </li> <li>have to modify the input string in any way (like getting the substring at index 8 and then using ^ in the regex)</li> <li>search the rest of the string for a match or do anything else that wouldn't be performant for a large number of tests against a large string.</li> </ol> <p>I would like to parse a potentially large user supplied body of text using an arbitrary user supplied grammar. The grammar will be defined in a BNF or PEG like syntax, and the terminals will either be string literals or regular expressions. Thus I will need to check if the next part of the string matches any of the potential terminals as driven by the grammar. </p>

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