In Python, how to check if a string only contains certain characters?
I need to check a string containing only a..z, 0..9, and . (period) and no other character.
I could iterate over each character and check the character is a..z or 0..9, or . but that would be slow.
I am not clear now how to do it with a regular expression.
Is this correct? Can you suggest a simpler regular expression or a more efficient approach.
#Valid chars . a-z 0-9
def check(test_str):
import re
#http://docs.python.org/library/re.html
#re.search returns None if no position in the string matches the pattern
#pattern to search for any character other then . a-z 0-9
pattern = r'[^.a-z0-9]'
if re.search(pattern, test_str):
#Character other then . a-z 0-9 was found
print 'Invalid : %r' % (test_str,)
else:
#No character other then . a-z 0-9 was found
print 'Valid : %r' % (test_str,)
check(test_str='abcde.1')
check(test_str='abcde.1#')
check(test_str='ABCDE.12')
check(test_str='_-/>"!@#12345abcde<')
'''
Output:
>>>
Valid : "abcde.1"
Invalid : "abcde.1#"
Invalid : "ABCDE.12"
Invalid : "_-/>"!@#12345abcde<"
'''
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Answer
Final(?) edit
Answer, wrapped up in a function, with annotated interactive session:
>>> import re
>>> def special_match(strg, search=re.compile(r'[^a-z0-9.]').search):
... return not bool(search(strg))
...
>>> special_match("")
True
>>> special_match("az09.")
True
>>> special_match("az09.n")
False
# The above test case is to catch out any attempt to use re.match()
# with a `$` instead of `Z` -- see point (6) below.
>>> special_match("az09.#")
False
>>> special_match("az09.X")
False
>>>
Note: There is a comparison with using re.match() further down in this answer. Further timings show that match() would win with much longer strings; match() seems to have a much larger overhead than search() when the final answer is True; this is puzzling (perhaps it’s the cost of returning a MatchObject instead of None) and may warrant further rummaging.
==== Earlier text ====
The [previously] accepted answer could use a few improvements:
(1) Presentation gives the appearance of being the result of an interactive Python session:
reg=re.compile('^[a-z0-9.]+$')
>>>reg.match('jsdlfjdsf12324..3432jsdflsdf')
True
but match() doesn’t return True
(2) For use with match(), the ^ at the start of the pattern is redundant, and appears to be slightly slower than the same pattern without the ^
(3) Should foster the use of raw string automatically unthinkingly for any re pattern
(4) The backslash in front of the dot/period is redundant
(5) Slower than the OP’s code!
prompt>rem OP's version -- NOTE: OP used raw string!
prompt>python26python -mtimeit -s"t='jsdlfjdsf12324..3432jsdflsdf';import
re;reg=re.compile(r'[^a-z0-9.]')" "not bool(reg.search(t))"
1000000 loops, best of 3: 1.43 usec per loop
prompt>rem OP's version w/o backslash
prompt>python26python -mtimeit -s"t='jsdlfjdsf12324..3432jsdflsdf';import
re;reg=re.compile(r'[^a-z0-9.]')" "not bool(reg.search(t))"
1000000 loops, best of 3: 1.44 usec per loop
prompt>rem cleaned-up version of accepted answer
prompt>python26python -mtimeit -s"t='jsdlfjdsf12324..3432jsdflsdf';import
re;reg=re.compile(r'[a-z0-9.]+Z')" "bool(reg.match(t))"
100000 loops, best of 3: 2.07 usec per loop
prompt>rem accepted answer
prompt>python26python -mtimeit -s"t='jsdlfjdsf12324..3432jsdflsdf';import
re;reg=re.compile('^[a-z0-9.]+$')" "bool(reg.match(t))"
100000 loops, best of 3: 2.08 usec per loop
(6) Can produce the wrong answer!!
>>> import re
>>> bool(re.compile('^[a-z0-9.]+$').match('1234n'))
True # uh-oh
>>> bool(re.compile('^[a-z0-9.]+Z').match('1234n'))
False