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copied!<p>Use them where you need to use/manipulate <em>patterns</em>. For instance, suppose you need to recognise the following pattern:</p> <ul> <li>Any letter, A-Z, either upper or lower case, 5 or 6 times</li> <li>3 digits</li> <li>a single letter a-z (definitely lower case)</li> </ul> <p>(Things like this crop up for zip code, credit card, social security number validation etc.)</p> <p>That's not really <em>hard</em> to write in code - but it becomes harder as the pattern becomes more complicated. With a regular expression, you describe the <em>pattern</em> (rather than the code to validate it) and let the regex engine do the work for you.</p> <p>The pattern here would be something like </p> <pre><code>[A-Za-z]{5,6}[0-9]{3}[a-z] </code></pre> <p>(There are other ways of expressing it too.) Grouping constructs make it easy to match a whole pattern and grab (or replace) different bits of it, too.</p> <p>A few downsides though:</p> <ul> <li>Regexes can become complicated and hard to read quite quickly. Document thoroughly!</li> <li>There are variations in behaviour between different regex engines</li> <li>The complexity can be hard to judge if you're not an expert (which I'm certainly not!); there are "gotchas" which can make the patterns really slow against particular input, and these gotchas aren't obvious at all</li> <li>Some people overuse regular expressions massively (and some underuse them, of course). The worst example I've seen was where someone asked (on a C# group) how to check whether a string was length 3 - this is clearly a job for using String.Length, but someone <em>seriously</em> suggested matching a regex. Madness. (They also got the regex wrong, which kinda proves the point.)</li> <li>Regexes use backslashes to escape various things (e.g. use . to mean "a dot" rather than just "any character". In many languages the backslash itself needs escaping.</li> </ul>

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