What flavor of Regex does Visual Studio Code use?

Rust Regex in the Find/Replace in Files Sidebar

Rob Lourens of MSFT wrote that the file search uses Rust regex. The Rust language documentation describes the syntax.

JavaScript Regex in the Find/Replace in File Widget

Alexandru Dima of MSFT wrote that the find widget uses JavaScript regex. As Wicktor commented, ECMAScript 5's documentation describes the syntax. So does the MDN JavaScript Regular Expression Guide.

Test the Difference

The find in files sidebar does not support (?=foobar) whereas the find in file widget does support that lookahead syntax.

Regarding Find/Replace with Groups

To find/replace with groups, use parentheses () to group and $1, $2, $3, $n to replace.

Here is an example.

Before:

After:

Shaun's answer is still correct, however to add an update, recently VS Code added the option to opt into using the Perl based PCRE2 engine. You can enable this through your settings config.

This allows you to perform more advanced regex operations like lookaheads and backreferences. But as noted below, the regex still has to be valid JavaScript regex.

VS Code does support regular expression searches, however, backreferences and lookaround aren't supported by default. But you can enable these with the setting search.usePCRE2. This configures ripgrep to use the PCRE2 regex engine. While PCRE2 supports many other features, we only support regex expressions that are still valid in JavaScript, because open editors are still searched using the editor's JavaScript-based search.

And for a bonus if you ended up here trying to do multi line searches, VS Code recently added that feature as well!

I've found newer information (July 22, 2020) about it.

IllusionMH left the following comment in Github:

ripgrep (compatible with PCRE2) is already used for Find in files functionality (for not open editors) and JS engine used only for open editors.