in reply to Regular Expression Builder
Just a thought: replace all letters by "A" and all digits by "9". Then apply the Regex::PreSuf thing — or just quotemeta(). And in that result, replace "A" with '\w' and "9" with '\d'.
Intermediate steps, as an example: @foo23 -> @AAA99 -> \@AAA99 -> \@\w\w\w\d\d
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