Inconsistent regex between Live Preview and JFlex Generator

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!

Start by comparing how the Live Preview and JFlex Generator process the example regex `[$_\p{ID_Start}][$\p{ID_Continue}]*`, especially Unicode character classes. Reproduce the Live Preview annotation and the successful lexer generation; done means both paths accept the same expression without a syntax error.

Written by the indexing model from the issue text.