`metavariable-pattern` with `pattern-regex` doesn't match `_`

[Simmo Saan]

Describe the bug
metavariable-pattern with pattern-regex doesn't match _, while metavariable-regex with the same regex does.
The former also produces:

Internal matching error when running try-catch-all-bug on target.ml:
 An error occurred while invoking the Semgrep engine. Please help us fix this by creating an issue at https://github.com/returntocorp/semgrep

metavariable-pattern failed because $E does not bind to a sub-program, please check your rule Engine(MatchingError)

To Reproduce
Doesn't work: https://semgrep.dev/playground/s/L1qv9.
Works: https://semgrep.dev/playground/s/8GNPq.

Expected behavior
_ is matched in both cases, regardless of metavariable matching syntax.

What is the priority of the bug to you?

• P0: blocking your adoption of Semgrep or workflow
• P1: important to fix or quite annoying
• P2: regular bug that should get fixed

Use case
Write rule for catch-all exception handlers, regardless of whether they use a wildcard or an identifier.