This feature is worked on in features/embeddedRegex branch.

• common infrastructure (Extract out the common portions of the Json and Regex parsing subsystems. #25541)
• Regex parser (Add in a regex parser so that the IDE can provide services around regex editing. #23984)
• common infrastructure for language service (Extract out subsystem for allowing embedded languages to export language services. #25985)
• Language service (Add Regex Language Service features to the IDE. #25517)
• Tests (All the regex parsing tests in their own branch. #25520)
• Additional validation before merging

Do we want to offer completion?