SUMMARY
When the search string entered in the "Search and Replace" plugin is a regular expression with lookahead or lookbehind part, the replace functionality does not work.

STEPS TO REPRODUCE
1. Open Kate and write a simple text in the editor, for ex:
SRC0
SRC1
SRC2

2. Open Search and Replace, enable regular expression search and type the following regex in the search field:
SRC(?=\d)

3. Type any replace string in the replace field, for ex:
DST

3. Press Search. The found lines will display fine.

4. Press Replace or Replace Checked

OBSERVED RESULT
The found "SRC" strings are not replaced with "DST"

EXPECTED RESULT
The found "SRC" strings should be replaced with "DST"

SOFTWARE/OS VERSIONS
Observed in Kate 21.12.0 for Windows, as well as Kate in the unstable branch of KDE Neon.
Qt Version: 5.15.2 (for the Windows version)

ADDITIONAL INFORMATION
In KWrite (in KDE Neon unstable branch), the above steps work fine.