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javascriptregexuppercaselowercase

How do I match a regex group and its lowercase version?

This is an example of what the regex would and wouldn't match:

# Matches
AAAA: aaaa
# Matches
ABCD: abcd
# Doesn't match
AAAA: abcd
# Doesn't match
AAAA: AaAa

How can I accomplish this?

I found this, but it doesn't work for matches because \L transforms the matches in the replace. Besides, \L seems to be only available in PHP and not in Javascript:

This works, but only when the case-insensitive option is set and it matches the last example:

(\w+): \1
Solution

  • You might be able to use case-sensitivity switch and lookahead. eg.

    \b(?=[A-Z]+:\s*[a-z]+)(?i)(\w+):\s*\1\b

    or

    \b(?=\p{Lu}+:\s*\p{Ll}+)(?i)(\p{L}+):\s*\1\b

    Essentially you use 2 regexes at once.

    • The first (i.e. everything within (?=...)) asserts that the first word is all uppercase ([A-Z]+ or \p{Lu}+) and that the second word is all lowercase ([a-z]+ or \p{Ll}+).
    • Then you turn on case-insensitivity with (?i).
    • Then the second regex looks for 2 words that are equal (ignoring case).

    The \b prevent matches on input like: xxAAA: aaayy

    Note: As the question mentioned VSCode, this answer uses .NET-style regex and assumes that the i modifier is initially turned off but can be toggled. However, I don't think this is possible in ECMAScript ("flags are an integral part of a regular expression. They cannot be added or removed later").