Assign a regular expression a variable name and use it in another regular expression

Question

So in flex, I'm trying to assign these:

ID     letterletter*
NUM    digitdigit*
letter [a-zA-Z]
digit  [0-9]

But flex is telling me that the ID and NUM are unrecognized rules. One way to bypass this would be setting ID to [0-9][0-9]*, but I want to use letter and digit.

Is this be possible?

Answer 1

Try this:

ID     {letter}{letter}*
NUM    {digit}{digit}*
letter [a-zA-Z]
digit  [0-9]

The same goes for ID and NUM, anywhere you use them.