Metavariable syntax for python about pfff HOT 3 CLOSED

@nbrahms points out that patterns containing $foo don't parse: https://semgrep.dev/s/3xZG/

The metavariable syntax is checked by semgrep-core/parsing/Check_semgrep.ml (check_pattern_metavars), resulting in a runtime error as desired. Does the lexer accept lowercase $foo on purpose?

from pfff.

Not on purpose; I was lazy to introduce a new regexp for uppercase identifiers and also I preferred to have only
one place specifying the syntax of a metavariable as we can change that sometimes, so simpler to be more general in the Python lexer and catch the error later in Check_semgrep.ml.

from pfff.

I can fix Lexer_python.mll if it bothers you. I think right now the syntax for metavariable is pretty fixed.

from pfff.