Regular expression to confirm whether a string is a valid Python identifier?

Question

I have the following definition for an Identifier:

Identifier --> letter{ letter| digit}

Basically I have an identifier function that gets a string from a file and tests it to make sure that it's a valid identifier as defined above.

I've tried this:

if re.match('\w+(\w\d)?', i):     
  return True
else:
  return False

but when I run my program every time it meets an integer it thinks that it's a valid identifier.

For example

c = 0 ;

it prints c as a valid identifier which is fine, but it also prints 0 as a valid identifer.

What am I doing wrong here?

Answer 1

From official reference: identifier ::= (letter|"_") (letter | digit | "_")*

So the regular expression is:

^[^\d\W]\w*\Z

Example (for Python 2 just omit re.UNICODE):

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

tests = [ "a", "a1", "_a1", "1a", "aa$%@%", "aa bb", "aa_bb", "aa\n" ]
for test in tests:
    result = re.match(identifier, test)
    print("%r\t= %s" % (test, (result is not None)))

Result:

'a' = True
'a1'    = True
'_a1'   = True
'1a'    = False
'aa$%@%'    = False
'aa bb' = False
'aa_bb' = True
'aa\n'  = False