Regex Anchors Tester (^, $, \b, \B)

Regex anchors are zero-width assertions — they match positions, not characters. ^ = start of line. $ = end of line. \b = word boundary. The behavior changes with the /m (multiline) flag. This tester visualizes every anchor position in your input.

How to use the Regex Anchors Tester (^, $, \b, \B)

Pick an anchor. Toggle multiline mode. The output highlights every position where the anchor matches.

Anchors match positions, not characters

An anchor is a zero-width assertion: it matches a position between characters rather than consuming a character. ^ asserts the start of the string (or of a line with the m flag) and $ the end. \b sits on a word boundary — the seam between a \w character and a \W character or string edge — while \B matches everywhere that is not such a seam.

Because they consume nothing, anchors are how you say "this must be the whole value" with ^...$, or "this word, not a fragment of a longer one" with \bcat\b so it skips category. The multiline flag is the usual source of confusion, and seeing every match position highlighted makes its effect obvious.

Common use cases

• Whole-string validation — wrap a pattern in ^...$ so a partial match cannot sneak through.
• Whole-word search — use \b to match cat without matching concatenate.
• Multiline log scanning — with the m flag, ^ERROR finds the start of every failing line.
• Debugging stray matches — see exactly why a pattern fires mid-string when you expected the start.
• Trimming and splitting — anchor the edges before stripping leading or trailing tokens.