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Asked: 2026-06-06T22:05:50+00:00

I can never seem to find any documentation on the regex for matching a

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I can never seem to find any documentation on the regex for matching a capture group as part of the pattern. For example:

(\w\d\w):$1

..should match a4b:a4b

$1 doesn’t work, but I know it’s something similar. Anyone know?

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1 Answer

  1. Editorial Team

    In a regex pattern, a backreference to the first capturing group is always \1, not $1.

    Reason: $ means “end of string” (or end of line, depending on context) in a regex.

    In a replace pattern (which isn’t a regex), some dialects allow $1 (e. g. .NET, Java, Perl and JavaScript), some allow \1 (Python and Ruby), and some allow both (PHP and JGSoft).

    Edit: Since you wrote that you couldn’t find any documentation on this, check out these overviews on regular-expressions.info:

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