I want to use RegEx to match characters in a sequence that only appear one time within that word. So for example in armored armadillo the only matches would be e, i and .
My first attempt was to use lookaround to match characters such that no copy of the same character appeared before or after the match:
(.)(?<!^.*\1)(?!\1.*$)
But this appears to match no characters no matter what I do. What am I doing wrong? How can I match characters in the way that I want to?
You can use
(.)(?<!\1.+)(?!.*\1)
The regex captures a char into Group 1 and then makes sure there is no such char before and after with the two lookarounds.
To match any chars including line breaks replace . with [\s\S] or prepend the pattern with (?s) inline modifier ((?m) in Ruby).
Details
(.) - Group 1: any single char (other than line break char by default)(?<!\1.+) - a negative lookbehind that fails the match if there is Group 1 value followed by one or more chars other than line break chars as many as possible immediately to the left of the current position (+ is required here to make sure . matches at least one char, the one that was captured into Group 1)(?!.*\1) - a negative lookahead that fails the match if there are zero or more chars other than line break chars as many as possible and then Group 1 value immediately to the right of the current position.See the regex demo.