Is there a way to make a Terminal match every NAME except for specific keywords?

Question

I am using lark to parse some text and needed a way to match a NAME that did not have certain keywords in it. I have the keywords listed out in a terminal I am just not sure how to make the terminal I need using it.

Here is the way I formatted my keywords

keywords: "var"
        | "let"
        | "type"

All help on this is appreciated!

Answer 1

Lark has a built-in support for the concept of keywords. So, it is unlikely that you need to explicitly exclude keywords NAME .

For example:

l = Lark("""
    %import common (LETTER, DIGIT)
    NAME: LETTER (LETTER | DIGIT)*
    keywords: "var"
            | "let"
            | "type"

    start: NAME | keywords
""", parser="lalr")

print(l.parse("hello"))     # Tree('start', [Token('NAME', 'hello')])
print(l.parse("let"))       # Tree('start', [Tree('keywords', [])])

Having said that, if you must, you can accomplish this by using a regexp:

l = Lark("""
    %import common (LETTER, DIGIT)
    NAME: /(?!(let|type|var))/ LETTER (LETTER | DIGIT)*
    start: NAME
""")

print(l.parse("hello"))     # Tree('start', [Token('NAME', 'hello')])
print(l.parse("let"))       # Exception, terminal not defined

PS keep in mind that "TERMINAL" is upper-case, and "rule" is lower-case, and they have behave differently in Lark, so it's important to keep the distinction in mind.