What flavor of Regex does Visual Studio Code use? - regex

Question

Trying to search-replace in Visual Studio Code, I find that its Regex flavor is different from full Visual Studio. Specifically, I try to declare a named group with string (?<p>[\w]+) which works in Visual Studio but not in Visual Studio Code. It'll complain with the error Invalid group.
Apart from solving this specific issue, I'm looking for information about the flavor of Regexes in Visual Studio Code and where to find documentation about it, so I can help myself with any other questions I might stumble upon.
Full Visual Studio uses .NET Regular Expressions as documented here. This link is mentioned as the documentation for VS Code elsewhere on Stackoverflow, but it's not.

Answer 1

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:

Answer 2

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!

Answer 3

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.

Answer 4

Which regex engine does vscode use? It is now a little more nuanced than previously. The best source is this vscode wiki: Github wiki: Notes on Regular Expression Support:
[At top of document]
This document applies to search (CMD+SHIFT+F/CTRL+SHIFT+F) and
quickopen (CMD+P/CTRL+P). By default, VS Code uses the ripgrep tool to
drive search.
...
[At end of document]
Text search uses two different sets of regular expression engines. The
workspace is searched using ripgrep, which will use the Rust regex
engine, and will fallback to PCRE2 if the regex fails to parse in the
Rust regex engine. The Rust regex engine doesn't support some features
like backreferences and look-around, so if you use those features,
PCRE2 will be used. Open files are searched using a JS regex in the
editor itself. Most of the time, you don't need to worry about this,
but you may see an inconsistency in how some complex regexes are
interpreted, and this can be an explanation. Especially when you see a
regex interpreted one way when a file is open, and another way when it
is not. During a Replace operation, each file will be opened in turn,
and the search query will be run as a JS regex.
Another potential issue is how newlines are handled between ripgrep
and the editor. The editor normalizes newlines, so that you can match
both CRLF and LF line endings just with \n. It's actually not possible
to match \r explicitly in the editor because it is normalized away.
When searching in the workspace, VS Code tries to rewrite a regex so
that \n will match CRLF. But \r\n or \s\n will also match CRLF in
closed files, but not in open files.
Two key points: (1) newlines are handled specially and (2) backrefereences and look-arounds are supported despite using the Rust regex engine - if your regex has a look-around or backreference in it PCRE2 will be used instead of the Rust engine.
More on lookarounds
The Find Widget (Ctrl+F) used for finding within the active editor only supports all lookarounds (lookahead and lookbehind) and those lookarounds can be non-fixed-length. So this will work in the Find Widget: (?<!blah.*).
In a search across files (Ctrl+Shift+F) non-fixed-length lookbehinds DO NOT WORK. Lookaheads can be fixed or non-fixed-length. But non-fixed-length positive or negative lookbehinds do not work and you will get an error message below the search input box: Regex parse error: lookbehind assertion is not fixed length which may not appear until you actually try to run the search.