Why does (.*)* make two matches and select nothing in group $1?

Question

This arose from a discussion on formalizing regular expressions syntax. I\'ve seen this behavior with several regular expression parsers, hence I tagged it language-agnostic.

Answer 1

Let's see what happens:

1. (.*) matches "input".
2. "input" is captured into group 1.
3. The regex engine is now positioned at the end of the string. But since (.*) is repeated, another match attempt is made:
4. (.*) matches the empty string after "input".
5. The empty string is captured into group 1, overwriting "input".
6. $1 now contains the empty string.

A good question from the comments:

Then why does replace("input", "(input)*", "A$1B") return "AinputBAB"?

1. (input)* matches "input". It is replaced by "AinputB".
2. (input)* matches the empty string. It is replaced by "AB" ($1 is empty because it didn't participate in the match).
3. Result: "AinputBAB"