The placeholder-restore pass in mdToHtml put code, math, mermaid and
allowed-HTML blocks back with a string replacement, so String.replace read
`$&`, `` $` ``, `$'` and `$$` in the *replacement* as substitution patterns.
A fenced block containing them rendered corrupted: `$&` re-inserted the
placeholder (`perl -pe 's/world/$& again/'` became
`s/world/___CODE_BLOCK_0___amp; again/`), `` $` `` and `$'` spliced in the
surrounding document, and `$$` collapsed to a single `$`.
Pass a function replacer at all four sites, matching the inline-code site
below them, which was already fixed this way. A function's return value is
inserted verbatim with no `$` interpretation.
The inline-code comment claimed `echo $1` would be read as a back-reference;
with a string search value there are no capture groups, so `$1` is already
literal. Reworded to name the four sequences that do corrupt.
Fixes#5663
Husam
578312200a