Because character classes are determined when the regex is compiled (which is different than when the Perl statement that contains the regex is compiled). There is no regex 'node' for "character class that consists of these hard-coded characters plus the characters in this backreference". The only character class regex node type is "hard-coded list of characters" that was built when the regex was compiled (not after it ran part way and figured out what $1 might end up being).

                - tye

In reply to Re^3: Regex backreference problem. (compile-time) by tye
in thread Regex backreference problem. by BrowserUk

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