BUG: Fix regression on DataFrame.replace for regex
The commit ensures that the replacement for regex is not confined to the 
beginning of the string but spans all the characters within. The 
behaviour is then consistent with versions prior to 0.24.0.

- [x] closes #25259
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
