Support Unicode RegExp property escapes

TypeScript currently doesn't transpile Unicode property escapes (of the form \p{ID_Start} or \P{ASCII}) in regular expressions.

It would be great if it did!

https://www.typescriptlang.org/play/?target=1#code/MYewdgzgLgBATgUwOYIB4wLwwPQB0AOA3gMrBwCW+UGA4oggNYC+2ArgNwBQokIANggB0fEEgAUiFKkFQE0MQHJAA8AKAlKvZA

Search Terms

regexp, regular expression, Unicode, property escapes, ES2018

Suggestion

Support transpiling Unicode property escapes in regular expressions. Examples:

/\p{ID_Start}/u;
/\P{ASCII}/u;
/\p{Script_Extensions=Greek}/u;

Use Cases

One particular use case is matching identifier characters in JavaScript parsers. This is currently commonly implemented as a large script-generated regular expression pattern (like in Esprima) or as a magical-looking list of code point ranges (like in TypeScript itself). However, it would be much simpler to use property escapes.

const regexIdentifierStart = /[$_\p{ID_Start}]/u;
const regexIdentifierPart = /[$_\u200C\u200D\p{ID_Continue}]/u;
const regexIdentifierName = /^(?:[$_\p{ID_Start}])(?:[$_\u200C\u200D\p{ID_Continue}])*$/u;

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reproducing the listed Unicode property escape examples in the linked TypeScript Playground with the stated target. Investigate the compiler's regular-expression transpilation path and determine how the examples should be emitted for older targets. Done means the examples transpile correctly without changing their runtime behavior.