Inconsistent regex between Live Preview and JFlex Generator

[siketyan]

Regular expressions are incosistent between Live Preview and JFlex Generator.
Especially some Unicode character classes are not supported in Live Preview.

For example:

IDENT = 'regexp:[$_\p{ID_Start}][$\p{ID_Continue}]*'

IDE says it's an unknown character category.

It's not recognised on Live Preview, annotating there's a syntax error.

However, generating lexer (from generated flex file from the bnf) completes successfully.

The regex works in the plugin as well!