Lookahead assertions (regular expressions) not working in JFlex

Question

I am trying to generate a scanner using JFlex.

One of the identifier rules for the grammar I am trying to implement is as follows (in regex syntax):

[a-zA-Z]((_(?!_)|[a-zA-Z0-9])*[a-zA-Z0-9])?

I have verified that the above expression is actually valid and works. However, it seems jFlex 1.4.3 does not like my use of the lookahead assertion (?!_) as it keeps complaining of the "question mark". Is there something I am doing wrong or is there a different to use lookaheads in JFlex?

I am using the "lookahead" to satisfy the rule that says I cannot have two consecutive underscores in the identifier name.

Any help would be appreciated.

Answer 1

Instead of

[a-zA-Z]((_(?!_)|[a-zA-Z0-9])*[a-zA-Z0-9])? 

use optimized pattern

[a-zA-Z]+(_[a-zA-Z0-9]+|[a-zA-Z0-9]+)*