How to write a regular expression to check whether an input is an identifier or not

Question

I have an identifier function that gets a string from a file and tests it to make sure that it's a valid identifier. So how do I write a regular expression to confirm the same?

Answer 1

As per the documentation, an identifer can be defined as - identifier ::= (letter|"_") (letter | digit | "_")*

So the regular expression is  - ^[^\d\W]\w*\Z. You can refer the following code to understand a bit more.

import re
identifier = re.compile(r"^[^\d\W]\w*\Z", re.UNICODE)

tests = [ "c", "b11", "_z1", "1q", "da$%@%", "xx yy", "pp_ww", "vv\n" ]
for test in tests:
    result = re.match(identifier, test)
    print("%r\t= %s" % (test, (result is not None)))

The output will be like

'c' = True

'b11' = True

'_z1' = True

'1q' = False

'da$%@%' = False

'xx yy' = False

'pp_ww' = True

'vv\n' = False