Add verifiable examples, PCRE2 runner, and GitHub Actions

[xuzongbao]

Phase 4: make the cheat sheet’s ✓ / ✗ examples automatically checkable without turning the repo into an app.

Flavor choice

CI uses Python 3 + the pcre2 package (bundled libpcre2) on ubuntu-latest, not Python’s stdlib re and not JavaScript RegExp.

• The notes are PCRE-first; pcre2 can run the documented named-group form (?<Word>…) + \k<Word>, inline (?i), and (?#…).
• The checker compiles with ASCII by default so \w / \d / \b match the README’s PCRE default (no Han in \w).
• This is still not a full clone of regex101’s PCRE2 UI (unbounded capturing lookbehind is skipped; see below).

PyPI regex was considered (option B) but it rejects \k<name>. JS / RE2 cannot honestly execute the PCRE teaching cases (backreferences, named \k, comments). Perl is on the runner images but would need extra YAML modules.

Cases

examples.yml has 51 cases drawn from the README cookbook and core sections:

• 49 executed (metacharacters, quantifiers, character classes, alternation, groups / IPv4, negation, backreferences, lookaround, greedy vs lazy first/all, flags, email / CN mobile / date / URL-ish / integers-decimals)
• 2 skipped (skip: true):
  • redos-nested-plus-doc-only — ^(a+)+$ documented only, not timed in CI
  • lookaround-var-lookbehind-html — unbounded capturing lookbehind; the alternative <(\w+)>(.*?)</\1> is executed

Local run on this agent: passed: 49 skipped: 2 failed: 0.

How to run

python3 -m pip install -r requirements-ci.txt
python3 scripts/check_examples.py

Details and the flavor caveat: README section 如何运行校验. LICENSE is unchanged.