# POSIX Extended Regular Expression Syntax (Quick Reference)

This article lays out POSIX Extended Regular Expression syntax implemented by ThousandEyes for page content verification in HTTP Server tests. For those wanting a more comprehensive article on this regular expression syntax, along with testing and troubleshooting guidelines, see [this article](https://docs.thousandeyes.com/product-documentation/internet-and-wan-monitoring/tests/http-server-tests/posix-extended-regular-expression-syntax).

## Single-Character Expressions

![](https://1112912342-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-M4QARF6s57qxMrOHDTZ%2Fuploads%2Fgit-blob-4bcb79722323dc54945abd95cfa1b6d265cdbb9a%2Fsingle-character-expressions.png?alt=media\&token=a3f627bd-64ab-4d98-91ed-d26fecf6b393)

## Repeaters

| {n}   | previous expression matches exactly n times                   |
| ----- | ------------------------------------------------------------- |
| {n,}  | previous expression matches at least n times                  |
| {,m}  | previous expression matches at most m times                   |
| {n,m} | previous expression matches between n and m times (inclusive) |
| \*    | previous expression matches 0 or more times                   |
| ?     | previous expression matches 0 or 1 times                      |
| +     | previous expression matches 1 or more times                   |

## Patterns

| ( | start of pattern |
| - | ---------------- |
| ) | end of pattern   |

## Boundaries

| ^    | beginning of line (only valid when used at the beginning of a pattern) |
| ---- | ---------------------------------------------------------------------- |
| $    | end of line (only valid when used at the end of a pattern)             |
| \\\` | beginning of input (start of the page)                                 |
| '    | end of input (end of the page)                                         |
| \b   | word boundary                                                          |