Regular expressions, also known as patterns, are formulas that describe text strings using a specially designed syntax.

There are different kinds of regular expressions, usually associated with the programming language that uses them. The most common ones are Perl and Java regular expressions. Although they are very similar, small differences in syntax can lead to very different results when matching text against the same patterns.

SRX (Segmentation Rules eXchange) uses a subset of ICU (Internationalization Components for Unicode) regular expressions. The allowed subset is described in SRX's specification document.