# Regex Anchors and Boundaries

> Anchors match a position, not a character: the start or end of the string, or the edge of a word. They are the difference between a pattern that matches anywhere and one that matches only where you mean. This covers ^, $, \b, and their multiline behavior, plus the mistakes they cause.

Source: https://ronutz.com/en/learn/regex-anchors-and-boundaries  
Updated: 2026-07-01  
Related tools: https://ronutz.com/en/tools/regex

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Most of a regex matches characters. Anchors are different: they match a **position between characters**, and they consume nothing. That is what lets you say "at the start of the line" or "at the edge of a word" instead of just "somewhere."

## The common anchors

- **`^`** matches the start of the string, and **`$`** matches the end. `^abc$` matches only the exact string `abc`, not `xabcx`. Without them, a pattern is allowed to match any substring, which is the single most common cause of a regex matching more than intended.
- **`\b`** matches a **word boundary**, the position between a word character and a non-word character (or the start or end of the string). `\bcat\b` matches `cat` as a whole word but not the `cat` inside `category`. **`\B`** is the opposite: a position that is *not* a word boundary.
- Some flavors also offer **`\A`** and **`\z`** for the absolute start and end of the string, which matter once multiline mode is involved.

## Multiline changes what ^ and $ mean

By default `^` and `$` anchor to the whole string. Turn on **multiline mode** (the `m` flag) and they anchor to the start and end of each *line* instead, matching at every newline. This is what you want when scanning a block of text line by line, and a surprise when you did not expect it, because a pattern that matched once now matches on every line.

## Where they bite

Two mistakes recur. First, putting `^` or `$` in the middle of a pattern by accident, which can only match at a line edge and so usually matches nothing. Second, assuming `\b` understands your alphabet: in many engines a word character is ASCII only, so `\b` lands in the wrong place around accented letters or other scripts unless you enable Unicode mode. When a "whole word" match behaves oddly on non-English text, the word-boundary definition is the first thing to check.