A placeholder-dependent search_regexp was recompiled for every matching
header value. Compile it once per replacement operation, keep skipping
invalid expanded expressions, and merge the duplicated "*" code path.
Addresses scan report 6c26a7b4c2e5ddc3.
Code written by GitHub Copilot (GPT-6.1 Sol, Claude Opus 5.5); reviewed by mholt.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>