regex: `\p{Surrogate}` Unicode property rejected ('invalid pattern') — Rust regex crate has no surrogate scalar values; blocks string-width/ink

[proggeramlug]

Summary

The Unicode property escape \p{Surrogate} (general category Cs) is rejected at regex-compile time: new RegExp('\\p{Surrogate}', 'u') → SyntaxError: Invalid regular expression: /\p{Surrogate}/: invalid pattern. Every other property in the same family works.

Root cause: Perry compiles JS regexes to the Rust regex crate, which matches over Unicode scalar values (UTF-8) — surrogate code points (U+D800–U+DFFF) can't occur there, so the crate rejects the Surrogate/Cs property outright instead of treating it as a never-matching class.

This is the next ink wall after #4877 (Segmenter). string-width/index.js builds two regexes at module top-level that include \p{Surrogate}, so importing string-width (→ ink) throws at init before any user code:

// node_modules/string-width/index.js:19
const zeroWidthClusterRegex   = /^(?:\p{Default_Ignorable_Code_Point}|\p{Control}|\p{Format}|\p{Mark}|\p{Surrogate})+$/v;
// :22
const leadingNonPrintingRegex = /^[\p{Default_Ignorable_Code_Point}\p{Control}\p{Format}\p{Mark}\p{Surrogate}]+/v;

Isolation (exactly one token is at fault)

OK    /\p{Control}/u
OK    /\p{Mark}/u
OK    /\p{Format}/u
OK    /\p{Default_Ignorable_Code_Point}/u
OK    /[\p{Control}]/v                         ← the /v (unicodeSets) flag itself is fine
FAIL  /\p{Surrogate}/u                         → invalid pattern
FAIL  /^(?:…|\p{Surrogate})+$/u                → invalid pattern (same with /v)

So it's specifically \p{Surrogate} — not the /v flag, not the other properties.

Expected (Node)

/\p{Surrogate}/u compiles and matches lone surrogate code units ("\uD800"). Node/V8 matches per UTF-16 code unit, so surrogates are matchable.

Suggested fix

In the JS→Rust regex translation (crates/perry-runtime/src/regex/compile.rs / grammar.rs), special-case \p{Surrogate} / \p{gc=Cs} / \P{Surrogate} rather than passing it through to the Rust crate:

• Minimum (unblocks ink): rewrite \p{Surrogate} to a never-matching subexpression (e.g. (?!) / empty class) and \P{Surrogate} to "any". For valid (non-surrogate) input — which is all string-width ever sees — this is behavior-preserving, and these regexes only ever test for zero-width/ignorable clusters.
• Faithful (matches V8): since Perry already carries WTF-8 / lone-surrogate support (String representation: support lone surrogates (WTF-8) for UTF-16 semantics #4793 / fix(runtime): round-trip lone surrogates via WTF-8 (#4793) #4806), map \p{Surrogate} to the code-unit range [\u{D800}-\u{DFFF}] against the WTF-8 representation so lone surrogates actually match. Heavier; not needed for the ink path.

Impact

• string-width@7+ → wrap-ansi, cli-truncate, slice-ansi → ink end-to-end (Compile ink (React-based TUI framework) end-to-end via perry.compilePackages #348). After this, ink's next gate is yoga-layout's WASM runtime (the documented out-of-scope rock).
• Any package using \p{Surrogate} for input sanitization / width calculation.
• Sits at the intersection of the two known categorical gaps in CLAUDE.md (Rust regex crate limits + lone-surrogate/WTF-8 handling), but is a narrow, self-contained translation fix.

Related

• Compile ink (React-based TUI framework) end-to-end via perry.compilePackages #348 (ink end-to-end smoke test — where this surfaced)
• globals/Intl: implement Intl.Segmenter (grapheme segmentation) — new Intl.Segmenter() throws, blocks string-width/wrap-ansi/ink #4877 / feat(intl): implement Intl.Segmenter (grapheme/word/sentence) — closes #4877 #4882 (previous ink wall: Intl.Segmenter)
• Codegen: new MessageChannel() global constructor unlinked/non-constructible — routes to stdlib symbol, breaks React scheduler init #4873 / fix(hir): global new MessageChannel() routes to always-linked runtime constructor (#4873) #4875 (new MessageChannel())
• String representation: support lone surrogates (WTF-8) for UTF-16 semantics #4793 / fix(runtime): round-trip lone surrogates via WTF-8 (#4793) #4806 (existing WTF-8 / lone-surrogate support)