In Python, How to Check If a String Only Contains Certain Characters

In Python, how to check if a string only contains certain characters?

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>\python26\python -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>\python26\python -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>\python26\python -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>\python26\python -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('1234\n'))
True # uh-oh
>>> bool(re.compile('^[a-z0-9\.]+\Z').match('1234\n'))
False

Check if a string contains only given characters

You could use any with a generator expression:

if any(c not in 'abc' for c in _str):  # Don't use str as a name.
    print('Wrong character')

Check if a string contains only characters from a regular expression

this reproduces what you want: ^ matches the beginning of the string $ the end. in between there are repeating + characters \w = [A-Za-z0-9_].

legal_characters = '^\w+$'

update

after the modification of your question this is my suggestion:

^ matches the beginning of the string $ the end. in between there are repeating + elements of [*-]:

legal_characters = '^[*-]+$'

there is no need to escape *- with \.

as pointed out by Maroun Maroun you can leave out the ^ as match scans the beginning of the string anyway:

legal_characters = '[*-]+$'

Test if string ONLY contains given characters

Assuming the discrepancy in your example is a typo, then this should work:

my_list = ['aba', 'acba', 'caz']
result = [s for s in my_list if not s.strip('abc')]

results in ['aba', 'acba']. string.strip(characters) will return an empty string if the string to be stripped contains nothing but characters in the input. Order of the characters should not matter.

How to check if a string contains only specific characters in python?

There are a couple of issues here. First . and - are meta-characters, and need to escaped with a \. Second, you don't really need the loop here - add a * to indicate any number of these characters, and qualify the regex between ^ and $ to signify the entire string needs to be made up of these characters:

import re
e = 'p12/5@gmail.com'
p = re.compile(r'^[a-zA-Z0-9@\.\-_]*$')

if p.match(e):
    print('okay')
else:
    print('n')

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