\1 provides the ability to substitute the literal text string of a previously matched parameter into the regular expression. It doesn't give you the ability to recompile the regular expression. Character class ([...]) are a compile time directive. \1 is a run time directive. [\1] is interpretted as 'the character with value 1'.

In reply to Re: Pattern matching: Why no \1 in [ ]? by MarkM
in thread Pattern matching: Why no \1 in [ ]? by Not_a_Number

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