Regular expressions are a feature common to a lot of technical programs which work with lots of text. Most programming languages also incorporate them as a feature to use.
Regular expressions are (usually) short pieces of text (strings) which describe patterns of text. These patterns can be used to identity text which conform to them. When this happens, the string is said to match the pattern. In this way, unknown text can be scanned for patterns, ranging from very simple (a letter or number) to dramatically complex (URLs, e-mail addresses, phone numbers, and so on).
One common use of regular expressions is to cull down a body of text to just that which matches a pattern—in other words, to find something. Another common use is to search and replace. Yet another is to validate text: You (or your program) may only care about text which matches a pattern, and all other text is irrelevant.
A regular expression, like I said, is essentially itself a short piece of text. Often, it's written in a spe