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Asked: 2026-06-11T07:18:14+00:00

What does [[]] mean in regex? $ echo ‘[][]’ | grep -oE ‘[[]]’ []

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What does [[]] mean in regex?

$ echo '[][]' | grep -oE '[[]]'
[]
[]
$ grep --version
grep (GNU grep) 2.10

Hmm, it appear that it matches [].
(The character sequences [], not [ or ].)
(I’ve tested it with python’s re module, same result.)
Really?
If so, why?

I knew if I want to match [ or ], I should have written [][] or [[\]].
(They work in PCRE, grep supports [][] but not [[\]] since \ loses special meaning in grep’s bracket expression.)
I’m only feeling curiosity.

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1 Answer

  1. Editorial Team

    The expression [[]] actually consists of two concatenated subexpressions: [[] and ].

    • [[] is character class that matches only [ characters. Having [ is only possible at the very beginning of a character class.
    • ] is just a normal character if outside of a character class.

    Both are concatenated thus your expression matches any character of [ followed by ], which results in matching [].

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