Only accept alphanumeric characters and underscores for a string in python

Question

I'm currently writing validation code for a tool parameter in ArcMap 10 (updateMessages) and need to prevent users from using non-alphanumeric characters within a string as it will be used to name a newly created field in a feature class.

I have so far used 'str.isalnum()' however this of course excludes underscores. Is there an efficient way to only accept alphanumeric characters and underscores?

if self.params[3].altered:
  #Check if field name already exists
  if str(self.params[3].value) in [f.name for f in arcpy.ListFields(str(self.params[0].value))]:
    self.params[3].setErrorMessage("A field with this name already exists in the data set.")
  #Check for invalid characters
  elif not str(self.params[3].value).isalnum():
    self.params[3].setErrorMessage("There are invalid characters in the field name.")   
  else:
    self.params[3].clearMessage()

return

Answer 1

import re
if re.match(r'^\w+$', text):

Answer 2

Try regular expressions:

import re
if re.match(r'^[A-Za-z0-9_]+$', text):
    # do stuff

Answer 3

An alternative way, without using regular expression for this specific case :

if text.replace('_', '').isalnum():
   # do stuff

You can also check for ascii characters only :

if text.replace('_', '').isalnum() and text.isascii():
   # do stuff

Answer 4

If you are using Python3 and there are non-ASCII characters in your string it is better to compile the regex with 8-bit string setting.

import sys
import re

if sys.version_info >= (3, 0):
    _w = re.compile("^\w+$", re.A)
else:
    _w = re.compile("^\w+$")

if re.match(_w, text):
    pass

For more information please refer to here.