Python- how do I use re to match a whole string

27
votes

I am validating the text input by a user so that it will only accept letters but not numbers. so far my code works fine when I type in a number (e.g. 56), it warns me that I should only type letters and when I type in letters it doesn't return anything (like it should do). My problem is that it accepts it when I start by typing letters followed by numbers e.g. (s45). what it does is accept the first letter but not the whole string. I need it to accept the whole string.

def letterCheck(aString):
    if len(aString) > 0:
        if re.match("[a-zA-Z]", aString) != None:
            return ""
    return "Enter letters only"
4
pythonstringmatch
Well you have return empty, and try adding + to the end of your regular expression. - squiguy
no, the return part is fine. its meant to return nothing if it passes the validation test. - Thomas
Why are you not just using aString.isalpha()? - kindall
That's not the problem, but it's a fairly bad design IMO; return a boolean and print the error message in the calling code if necessary... - Wooble
The question title is misleading, solutions address the problem described which is not how to match a whole string. - kiril

4 Answers

50
votes

Anchor it to the start and end, and match one or more characters:

if re.match("^[a-zA-Z]+$", aString):

Here ^ anchors to the start of the string, $ to the end, and + makes sure you match 1 or more characters.

You'd be better off just using str.isalpha() instead though. No need to reach for the hefty regular expression hammer here:

>>> 'foobar'.isalpha()
True
>>> 'foobar42'.isalpha()
False
>>> ''.isalpha()
False
5
votes

use boundaries in your regex + raw string to encode the regex, like this:

r"^[a-zA-Z]+$"
3
votes

You might consider using isalpha() on the string. It returns true if the string contains nothing but alphabetic characters, false otherwise.

if aString.isalpha():
   do something
else:
   handle input error
0
votes

if you look for pretty pythonic writings, go for isalpha and isdecimal :

str = u"23443434";
print str.isdecimal();