Understanding Regular Expressions

The filtering examples in the previous two chapters enabled you to locate data using matches, comparisons, and wildcard operators. For basic filtering (and even some not-so-basic filtering) this might be enough. But as the complexity of filtering conditions grows, so does the complexity of the WHERE clauses themselves.

And this is where regular expressions become useful. Regular expressions are special strings (sets of characters) that are used to match text. If you needed to extract phone numbers from a text file, you might use a regular expression. If you needed to locate all files with digits in the middle of their names, you might use a regular expression. If you wanted to find all repeated words in a block of text, you might use a regular expression. And if you wanted to replace all URLs in a page with actual HTML links to those same URLs, yes, you might use a regular expression (or two, for this last example).

Regular expressions are supported in all sorts of programming languages, text editors, operating systems, and more. And savvy programmers and network managers have long regarded regular expressions as a vital component of their technical toolboxes.

Regular expressions are created using the regular expression language, a specialized language designed to do everything that was just discussed and much more. Like any language, regular expressions have a special syntax and instructions that you must learn.