In this tutorial, we will look at how to check if a string contains only letters and numbers in Python with the help of examples. You can use the string isalnum()
function to check if a string contains only letters (that is, alphabets) and/or numbers in Python. The following is the syntax – # check if string s contains only alphanumeric characters
s.isalnum() It returns True if all the characters in the string are alphanumeric (containing only alphabets and/or numbers). If any of the characters is not alphanumeric, it returns False. Let’s look at an example. # create a string
s = "Bunny123"
# check if string contains only alphanumeric characters
print(s.isalnum()) Output: True We get True as the output. This is because all the characters in the string s above are either
letters or numbers. Let’s look at another example. # create a string
s = "Bunny_123"
# check if string contains only alphanumeric characters
print(s.isalnum()) Output: False Here, we get False as the output. This is because not all characters in the string s above are alphanumeric. One of the characters in the string, “_” is neither an alphabet nor a digit. For more on the string isalnum() function, refer to its documentation. You might also
be interested in – - Python – Check If String Contains Only Letters
- Python – Check if String Contains Only Numbers
- Check If String Contains Only Certain Characters in Python
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Piyush is a data scientist passionate about using data to understand things better and make informed decisions. In
the past, he's worked as a Data Scientist for ZS and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects. View all posts
[Edit] There's another solution not mentioned yet, and it seems to outperform the others given so far in most cases. Use string.translate to replace all valid characters in the string, and see if we have any invalid ones left over. This is pretty fast as it uses the underlying C function to do the work, with very little python bytecode involved. Obviously performance isn't everything - going for the most readable solutions is probably the best approach when
not in a performance critical codepath, but just to see how the solutions stack up, here's a performance comparison of all the methods proposed so far. check_trans is the one using the string.translate method. Test code: import string, re, timeit
pat = re.compile('[\w-]*$')
pat_inv = re.compile ('[^\w-]')
allowed_chars=string.ascii_letters + string.digits + '_-'
allowed_set = set(allowed_chars)
trans_table = string.maketrans('','')
def check_set_diff(s):
return not set(s) - allowed_set
def check_set_all(s):
return all(x in allowed_set for x in s)
def check_set_subset(s):
return set(s).issubset(allowed_set)
def check_re_match(s):
return pat.match(s)
def check_re_inverse(s): # Search for non-matching character.
return not pat_inv.search(s)
def check_trans(s):
return not s.translate(trans_table,allowed_chars)
test_long_almost_valid='a_very_long_string_that_is_mostly_valid_except_for_last_char'*99 + '!'
test_long_valid='a_very_long_string_that_is_completely_valid_' * 99
test_short_valid='short_valid_string'
test_short_invalid='/$%$%&'
test_long_invalid='/$%$%&' * 99
test_empty=''
def main():
funcs = sorted(f for f in globals() if f.startswith('check_'))
tests = sorted(f for f in globals() if f.startswith('test_'))
for test in tests:
print "Test %-15s (length = %d):" % (test, len(globals()[test]))
for func in funcs:
print " %-20s : %.3f" % (func,
timeit.Timer('%s(%s)' % (func, test), 'from __main__ import pat,allowed_set,%s' % ','.join(funcs+tests)).timeit(10000))
print
if __name__=='__main__': main()
The results on my system are: Test test_empty (length = 0):
check_re_inverse : 0.042
check_re_match : 0.030
check_set_all : 0.027
check_set_diff : 0.029
check_set_subset : 0.029
check_trans : 0.014
Test test_long_almost_valid (length = 5941):
check_re_inverse : 2.690
check_re_match : 3.037
check_set_all : 18.860
check_set_diff : 2.905
check_set_subset : 2.903
check_trans : 0.182
Test test_long_invalid (length = 594):
check_re_inverse : 0.017
check_re_match : 0.015
check_set_all : 0.044
check_set_diff : 0.311
check_set_subset : 0.308
check_trans : 0.034
Test test_long_valid (length = 4356):
check_re_inverse : 1.890
check_re_match : 1.010
check_set_all : 14.411
check_set_diff : 2.101
check_set_subset : 2.333
check_trans : 0.140
Test test_short_invalid (length = 6):
check_re_inverse : 0.017
check_re_match : 0.019
check_set_all : 0.044
check_set_diff : 0.032
check_set_subset : 0.037
check_trans : 0.015
Test test_short_valid (length = 18):
check_re_inverse : 0.125
check_re_match : 0.066
check_set_all : 0.104
check_set_diff : 0.051
check_set_subset : 0.046
check_trans : 0.017
The translate approach seems best in most cases, dramatically so with long valid strings, but is beaten out by regexes in test_long_invalid (Presumably because the regex can bail out immediately, but translate always
has to scan the whole string). The set approaches are usually worst, beating regexes only for the empty string case. Using all(x in allowed_set for x in s) performs well if it bails out early, but can be bad if it has to iterate through every character. isSubSet and set difference are comparable, and are consistently proportional to the length of the string regardless of the data. There's a similar difference between the regex methods matching all valid characters and searching for
invalid characters. Matching performs a little better when checking for a long, but fully valid string, but worse for invalid characters near the end of the string.
How do you check if a string contains only letters and digits Python?
Check if a string is numeric, alphabetic, alphanumeric, or ASCII. Check if a string contains only decimal: str.isdecimal(). Check if a string contains only digits: str.isdigit(). Check if a string contains only numeric: str.isnumeric(). Check if a string contains only alphabetic: str.isalpha(). How do you check if a string only has letters and numbers?
Use the test() method on the following regular expression to check if a string contains only letters and numbers - /^[A-Za-z0-9]*$/ . The test method will return true if the regular expression is matched in the string and false otherwise.
How do you check if a string is number or alphabet in Python?
Python String isalpha() Method
The isalpha() method returns True if all the characters are alphabet letters (a-z). Example of characters that are not alphabet letters: (space)!
How do you check if a string contains only alphabets and spaces in Python?
Method #1 : Using all() + isspace() + isalpha()
This is one of the way in which this task can be performed. In this, we compare the string for all elements being alphabets or space only.
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