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copied!<p><strong>What regular expressions are used for:</strong></p> <p>Regular expressions is a language in itself that allows you to perform complex validation of string inputs. I.e. you pass it a string and it will return true or false if it is a match or not. </p> <p><strong>How regular expressions are used:</strong></p> <ul> <li>Form validation, determine if what the user entered is of the format you want</li> <li>Finding the position of a certain pattern in a block of text</li> <li>Search and replace where the search term is a regex and what to replace is a normal string. </li> </ul> <p><strong>Some regular expression language features:</strong></p> <ul> <li><p><strong>Alternation:</strong> allows you to select one thing or another. Example match only yes or no.</p> <p>yes|no</p></li> <li><p><strong>Grouping:</strong> You can define scope and have precedence using parentheses. For example match 3 color shades.</p> <p>gr(a|e)y|black|white</p></li> <li><p><strong>Quantification:</strong> You can quantify how much of something you want. ? means 1 or 0, * means 0 or more. + means at least one. Example: Accept a binary string that is not empty: </p> <p>(0|1)+</p></li> </ul> <p><strong>Why regular expressions?</strong> </p> <p>Regular expressions make it easy to match strings, it can often replace several dozen lines of source code with a simple small regular expression string. </p> <p><strong>Not for all types of matching:</strong> </p> <p>To understand how something is useful, you should also understand how it is not useful. Regular expressions are bad for certain tasks for example when you need to guarantee that a string has an equal number of parentheses. </p> <p><strong>Available in just about all languages:</strong> </p> <p>Regular expressions are available in just about any programming language. </p> <p><strong>Formal language:</strong></p> <p>Any regular expression can be converted to a deterministic finite state machine. And in this same way you can figure out how to make source code that will validate your regular expression. </p> <p><strong>Example:</strong> </p> <pre><code>[hc]+at </code></pre> <p>matches "hat", "cat", "hhat", "chat", "hcat", "ccchat", and so on, but not "at"</p> <p><a href="http://en.wikipedia.org/wiki/Regular_expression" rel="nofollow noreferrer">Source, further reading</a></p>

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