Can't convert RegExp:Unicode property escapes

[YOOYY]

There is a new feature in es2018, RegExp: Unicode property escapes; I tried converting it to es2015 but nothing changed in the code.

for example:

const regexGreekSymbol = /\p{Script_Extensions=Greek}/u;
console.log(regexGreekSymbol.test('π'));

output:

var regexGreekSymbol = /\p{Script_Extensions=Greek}/u;
console.log(regexGreekSymbol.test("\u03C0"));

this is my configuration:

{
  entryPoints: ['./demo.js'],
  outfile: 'outfile.js',
  target: ['es2015']
}

version: 0.14.38

[hyrious]

As far as I know there're some other new regex flags added recently. I wonder how could we polyfill/convert them correctly to lower environments. At least is there a reference implementation?

[evanw]

TypeScript's lowering transform doesn't change the RegExp literal either, even when targeting ES5 (link). See also microsoft/TypeScript#36132 and microsoft/TypeScript#32214. I'm guessing the most straightforward thing for esbuild to do here would be to convert regular expressions into new RegExp() to convert the syntax error into a run-time error. This would let you bring your own polyfill.

[YOOYY]

As far as I know there're some other new regex flags added recently. I wonder how could we polyfill/convert them correctly to lower environments. At least is there a reference implementation?

Babel seems to support converting RegExp.
I had this problem when using vite, so I had to add vite's babel plugin, I also saw esbuild's babel plugin on npm

[evanw]

Babel seems to support converting RegExp.

Not always. It looks like Babel just converts /./y into new RegExp(".", "y") for example. I assume that will crash in older browsers. Here's the relevant Babel thread: babel/babel#904. They are deliberately leaving this up to the polyfill to handle.

[jridgewell]

Babel can't implement sticky behavior without implementing a full matcher, so it's left to runtime. However, it's possible to support unicode, unicode properties escapes, unicode sets, named capture groups, and dotall through regexpu. For named capture groups, it requires a runtime wrapper, but everything else is just a syntax transform.

[evanw]

Thanks very much for the context about what Babel does handle. However, despite that I decided to fix this by converting unsupported regular expression literals to new RegExp(). One reason is that lowering regular expressions looks like a significant increase in complexity and this conversion is a much simpler way to solve this problem. Another reason is that a pure syntax conversion isn't totally correct, and you may still want/need a polyfill anyway (e.g. if you need the flags property to represent the original source code).