Regular expression working properly?

I only have half a handle on regular expressions. If I say:

replace the regular expression ".*" in R with "\0x"

where R is something like “abc”, should I expect “abcx” or “abcxx”?

I’m getting “abcxx,” which doesn’t seem right to me.

Note that

replace the regular expression "^.*$" in R with "\0x"

produces “abcx,” so that’s what I’ll use, but I’m curious about whether the first behavior is correct for reasons I don’t understand.

It is correct behavior, even though it’s not very intuitive.

The logic is that first “." matches all characters it finds and replaces them with “abcx”. Then it sees that nothing is left – but since ".” matches any character zero or more times, “nothing” is a valid match: it’s any character zero times. Therefore it replaces the “nothing” at the end with “x”, which causes you seeing two x’s in the end result.

“^.*$” is a good fix, or if R is never an empty string, “.+” works too (replace one or more characters so “nothing” is not a match anymore).

Interesting. That makes sense. Thank you.

You might try “.+” instead. The + means “1 or more”, where * is “0 or more” in regexes.