Regex look behind in VS Code?

I'm working on a grammar extension in VS Code, and I'm having difficulty with a look behind regex pattern. Given the following string, I want to only return cmp when it's preceded by the @fmt(

@fmt(cmp,foo)

The matching string I used in another editor was this:

(?<=[@|©](fmt)\()(\w+)

However, this is not working in VS Code, when I do a regex search it comes back with the error that it's not a valid expression. Playing around with it, the problem is the <= characters, which indicate the look behind.

Doing a search of the VS Code website doesn't return any kind of regex reference guide. Searching Stack Overflow came up with this question, which states that Visual Studio has a unique regex definitions. Unfortunately, the example given in that question doesn't work in VS Code.

Does anyone know how to do a look behind regex in VS Code? Or at least know where the regex documentation for VS Code is located?

I worry that it's not possible, since according to Stack Overflow reference look behinds aren't supported in JavaScript. There is another question that shows how to mimic look behinds in a JavaScript function, but I don't know if it's possible to extend a language in VS Code with user-defined functions. If anyone knows how to do that, and can point me in that direction, that would also be an acceptable workaround.

Solution 1:

You can use infinite-width lookahead and lookbehind without any constraint now beginning with Visual Studio Code v.1.31.0 release.

See proof:

and another one (with (?<=@fmt\([^()]*)\w+ pattern, note the * in the lookbehind):

See the Github [VSCode 1.31] ES2018 RegExp lookbehind assertions are now supported #68004 issue:

As a result of moving to Electron 3.0, RegExp lookbehind assertions are now supported, since they’re supported since Chromium 62 and Node 8.10.0, and Electron 3.0 uses Chromium 66 and Node 10.2.0, so they’re now supported, but the release notes don’t mention that lookbehind assertions are now supported.

VS Code developers confirm that it is true that they "forgot to mention it in the release notes".

Solution 2:

Update

Visual Studio Code v.1.31.0 and up have regex lookbehind support, see Wiktor's answer

Visual Studio code uses JavaScript's regular expressions as specified in ECMAScript 5, which doesn't support look behinds (https://github.com/Microsoft/vscode/issues/8635).

Here's a workaround you can try if you're doing find and replace (thanks cyborgx37):

Search Expression:

(lookbehind_expression_)text_to_replace

Replace Expression:

$1replacement_text

Given the input:

lookbehind_expression_text_to_replace

The output will be:

lookbehind_expression_replacement_text