> ## Knowledge Base Index
> Fetch the complete knowledge base index at: https://help.textit.com/sitemap.xml
> Use this file to discover available pages before exploring further.
> Pure-Markdown content can be obtained by appending a '.md' suffix to the content URLs listed in the sitemap (without the trailing slash).

# Invalid Regular Expressions

Regular Expressions, commonly referred to as *regexes*, can be used to build more advanced custom rules. Regexes need to conform to a strict syntax to ensure that they are well understood when they are evaluated. If you find yourself using this advanced form of rule matching, make sure you are familiar with how they are constructed.

We use Perl style rules for pattern matching. If you are interested, you can read more on the basic syntax we support [here](https://golang.org/pkg/regexp/syntax/).