fix(evaluate): serialize and parse RegExp values

[FadeHack (FadeHack)]

serialize_value() has no branch for re.Pattern, so passing a compiled pattern to evaluate() silently sends undefined. parse_value() has no branch for "r", so a RegExp coming back from the page reaches the caller as the raw wire dict instead of a pattern:

>>> page.evaluate("(v) => String(v)", re.compile(r"a\d+", re.I))
'undefined'
>>> page.evaluate("() => /a\\d+/gi")
{'r': {'p': 'a\\d+', 'f': 'gi'}}

The protocol already carries regexes as {r: {p, f}} and the driver implements both sides, so this just adds the two missing branches on the Python side. playwright-dotnet does the same in EvaluateArgumentValueConverter.cs.

Flags reuse the existing escape_regex_flags() helper. On the way back, the JavaScript-only flags (g, y, d, u, v) have no re equivalent and are dropped.

Fixes #3188

[FadeHack (FadeHack)]

@microsoft-github-policy-service agree

[Pavel Feldman (pavelfeldman)]

Thanks, this looks good. Reusing escape_regex_flags and adding parse_regex_flags is the right approach.

One concern on the parse side: re.compile raises for JS regex syntax that Python's re doesn't support. That now fails the whole evaluate() / json_value() call, even when the regex is nested deep inside a larger object. All of these raise re.error with this PR, while on main they return the raw dict:

page.evaluate("() => ({ re: /(?<year>\\d{4})/ })")  # unknown extension ?<y
page.evaluate("() => /[^]/")                          # unterminated character set
page.evaluate("() => /\\u{1F600}/u")                  # incomplete escape \u

Named groups are common in app code, so something like page.evaluate("() => window.appConfig") could start failing. Could you catch re.error in parse_value, fall back to the previous value, and add a test for it?

Minor: re.compile(b"a") fails with a protocol error (arg.value.r.p: expected string, got object). An explicit assert would give a clearer message.

[Pavel Feldman (pavelfeldman)]

Thanks for the fix and the investigation! Continuing this in #3225, which builds on your change and also handles regexes Python's re can't compile.