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POWhy are C# compiled regular expressions faster than equivalent string methods?
 

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  1. POWhy are C# compiled regular expressions faster than equivalent string methods?
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    <p>Every time I have to do simple containment or replacement operations on strings, where the term that I'm searching for is a fixed value, I find that if I take my sample input and do some profiling on it, using a compiled regular expression is nearly* always faster than using the equivalent method from the String class. </p> <p>I've tried comparing a variety of methods ( <code>hs</code> is the "haystack" to search, <code>ndl</code> is the "needle" to search for, <code>repl</code> is the replacement value. <code>regex</code> is always created with the <code>RegexOptions.Compiled</code> option ):</p> <ul> <li><code>hs.Replace( ndl, repl )</code> vs <code>regex.Replace( hs, repl )</code></li> <li><code>hs.Contains( ndl )</code> vs <code>regex.IsMatch( hs )</code></li> </ul> <p>I've found quite a few discussions focusing on <em>which</em> of the two techniques are faster (<a href="https://stackoverflow.com/questions/3186285/c-sharp-which-is-faster-string-contains-or-regex-ismatch">1</a>, <a href="https://stackoverflow.com/questions/3601465/string-split-vs-regex-split">2</a>, <a href="https://stackoverflow.com/questions/9380062/is-using-a-regular-expression-faster-than-indexof">3</a>, and loads of others), but those discussions always seem to focus on:</p> <ol> <li>Use the string version for simple operations and regex for complex operations (which, from a raw performance perspective, doesn't even seem to be necessarily a good idea), or</li> <li>Run a test and compare the two ( and for equivalent tests, the regex version seems to always perform better ).</li> </ol> <p>I don't understand how this can possibly be the case: how does the regex engine compare any two strings for substring matches faster than the equivalent string version? This seems to hold true for search spaces that are very small or very large, or search terms that are small or large, or whether the search term occurs early or late in the search space.</p> <p>So, <strong>why</strong> are regular expressions faster?</p> <hr> <p>* In fact, the <strong>only</strong> case I've managed to show that the string version is faster than a compiled regex is when searching an empty string! Any other case, from single character strings to very long strings are processed faster by a compiled regex than the equivalent string method.</p> <hr> <p><strong>Update:</strong> Added a clause to clarify that I'm looking at cases where the search term is known at compile time. For dynamic or one-time operations, the overhead of compiling the regular expression will tend to skew the results in favor of the string methods.</p>
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    1. USChris Phillips
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    1. COI believe your better question might be "If regex is faster, why don't the String methods just use regex?". Anyway, my only guess would be something involving either that 1. the regex compiling takes some additional time you're not accounting for, or 2. regex compiling/processing eats more memory or something, and you're getting a space-time tradeoff.
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      1. POWhy are C# compiled regular expressions faster than equivalent string methods?
      1. USzebediah49
    2. CO@zebediah49 I think you're right that my question leads into that question - but I'm really interested in the why or how regexes are performing better.
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    3. COWhere are the close votes coming from?
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