fix(sed): backreferences in search pattern fail (`\1` in match side)

[chaliy]

Summary

Sed patterns with backreferences (`\1`, `\2`, etc.) in the search/match side of a substitution fail with `backreferences are not supported`. Backreferences in the replacement side work correctly.

Reproduction

# Replacement backreference — WORKS:
echo "abc" | sed "s/a\(b\)c/\1/"
# Output: b

# Search-side backreference — FAILS:
echo "<a href='tag_hello.html'>hello</a>" | sed "s|<a href='tag_\([^']*\).html'>\1</a>|\1|g"
# Error: backreferences are not supported

The Rust `regex` crate does not support backreferences in patterns. This is a known limitation of the crate.

Context

Bashblog uses search-side backreferences in the `edit()` function to strip tag links back to plain text:

sed "/^<p>$template_tags_line_header/s|<a href='$prefix_tags\([^']*\).html'>\\1</a>|\\1|g"

This pattern captures the tag name, then uses `\1` in the search to ensure the tag text matches the href, then replaces the whole `` tag with just the tag name.

Possible approaches

1. Use `regex-lite` or `fancy-regex` crate which supports backreferences
2. Implement a custom backreference handler that does two-pass matching
3. Gracefully degrade: emit a warning and skip the substitution instead of crashing

Expected behavior

`sed` should support backreferences (`\1` through `\9`) in both the search and replacement portions of `s` commands, matching GNU sed behavior.