Python str.replace edits to LaTeX: watch the $ boundary

When applying a fail-loud old->new replacement list to LaTeX source, an old string that begins just after a closing $ (e.g. 'gives a peak of ...') leaves the math delimiter outside the match, so a new string starting with ', so ...' yields a stray space, and including '$ ' in old without '$' in new causes tectonic 'Missing $ inserted'. Anchor replacements so the same delimiters appear on both sides, and recompile plus grep the edited line after every replacement batch.