Mathematica regex equvalent of \l

Question

In normal RegEx \l will make the character to the right lowercase. However, it seems like the RegEx in Mathematica does not have this functionality.

For example

StringCases["Abba", RegularExpression["(.).*?\\l\\1"]]

will return the following error message

RegularExpression::msg37: "Mathematica does not support \\L, \\l, \\N, \\P, \\p, \\U, \\u, or \\X in RegularExpression[\!\(\"(.).*?\\\\l\\\\1\"\)]."

Is there some workaround that I can use? Note that (?i) is not sufficient as I need the character to be lowercase only.

Thanks

Answer 1

I believe this is what you want.

StringCases["Abba", RegularExpression["(.).*(?=(?i:\\1))[a-z]"]]

(?=(?i:\\1)) is a lookahead which says, "if the next character matches \1 in any case," but then it also has to match the pattern that occupies the place of the next character, which is [a-z] i.e. only lowercase letters.

If it fails to match the outer pair of Aa, it will match the inner pair of bb. I couldn't figure out if this was what you wanted or not, it does seem like it judging from your attempt.

Answer 2

This is not a regular expression (or PCRE), but it's a StringExpression that can be used where RegularExpression qualifies:

StringCases["Abba", 
 x : LetterCharacter ~~ Shortest[___] ~~ 
   y : LetterCharacter /; ToLowerCase[x] == y]

{"Abba"}

LetterCharacters are used instead of _ primarily to keep ToLowerCase meaningful.