Regex Explainer

Regex for a hex colour

Three or six hex digits, and why the alternation order matters here.

/^#(?:[0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/

Match a whole string consisting of "#", then (exactly 3 of "0" to "9", "a" to "f", or "A" to "F" or exactly 6 of "0" to "9", "a" to "f", or "A" to "F").

What to check

  • Note

    Anchors match string ends, not line ends

    Without the m flag, ^ and $ match only the very start and end of the input — not the start and end of each line. On multi-line input this is often not what was intended.

    Fix: Add the m flag if the pattern should apply per line.

Step by step

  1. ^

    Anchor to the start of the input.

  2. #

    Match "#".

  3. (?:[0-9a-fA-F]{3}|[0-9a-fA-F]{6})

    Match (exactly 3 of "0" to "9", "a" to "f", or "A" to "F" or exactly 6 of "0" to "9", "a" to "f", or "A" to "F").

  4. $

    Anchor to the end of the input.

Details

Capture groups
None
Flags
None set

Check your own pattern

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