Java String.replaceAll() with back reference

Question

There is a Java Regex question: Given a string, if the \"*\" is at the start or the end of the string, keep it, otherwise, remove it. For example:

1. *

Answer 1

You can use lookarounds in your regex:

String repl = str.replaceAll("(?

RegEx Demo

RegEx Breakup:

(?

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OP's regex is:

(^\*)|(\*$)|\*

It uses 2 captured groups, one for * at start and another for * at end and uses back-references in replacements. Which might work here but will be way more slower to finish for larger string as evident in # of steps taken in this demo. That is 209 vs 48 steps using look-arounds.

Another smaller improvement in OP's regex is to use quantifier:

(^\*)|(\*$)|\*+