Python program to verify that a string only contains letters, numbers, underscores and dashes

Given a string, we have to find whether the string contains any letters, numbers, underscores, and dashes or not. It is usually used for verifying username and password validity. For example, the user has a string for the username of a person and the user doesn’t want the username to have any special characters such as @, $, etc. 

Prerequisite: Regular Expression in Python

Let’s see the different methods for solving this task:

Method 1: Using Regular Expression.

There is a function in the regular expression library(re) that compares two strings for us. re.match(pattern, string) is a function that returns an object, to find whether a match is find or not we have to typecast it into boolean. 

Syntax:

re.match(pattern, string) 

Parameters:

• pattern: the pattern against which you want to check 
• string: the string you want to check for the pattern 

Return: Match object 

Let’s see an example:

Example 1:

Python3

# import library
import re

# make a pattern
pattern = "^[A-Za-z0-9_-]*$"

# input
string = "G33ks_F0r_Geeks"

# convert match object
# into boolean values
state = bool(re.match(pattern, string))

print(state)

Output:

True

Example 2:

Python3

# import library
import re

print(bool(re.match("^[A-Za-z0-9_-]*$",
                    'ValidString12-_')))

print(bool(re.match("^[A-Za-z0-9_-]*$",
                    'inv@lidstring')))

Output:

True
False

Method 2: Using Set.

Set is a built-in data type in Python. We are using issubset() function of a set that returns True if all characters of a set are present in a given set Otherwise False.

Syntax: 

set_name.issubset(set)

Parameters: 

• set: Represents that set in which the subset has to be searched 

Return: boolean value

Let’s see an example:

Example 1:

Python3

# create a set of allowed characters
allowed_chars = set(
    ("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"))

# input string
string = "inv@lid"

# convert string into set of characters
validation = set((string))

# check condition
if validation.issubset(allowed_chars):
    print("True")

else:
    print("False")

Output:

False

Example 2:

Python3

allowed_chars = set(
    "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-")

string = "__Val1d__"

validation = set((string))

if validation.issubset(allowed_chars):
    print("True")

else:
    print("False")

Output:

True

The Time and Space Complexity for all the methods are the same:

Time Complexity: O(n)

Auxiliary Space: O(n)

Method : Using replace() and len() methods

Python3

allowed_chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"

string = "__Val1d__"
for i in allowed_chars:
    string = string.replace(i, "")
if len(string) == 0:
    print("True")
else:
    print("False")

Output

True

Method : Using operator.countOf() method

Python3

import operator as op
# create a string of allowed characters
allowed_chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"

# input string
string = "inv@lid"

res = True

for i in string:
    if op.countOf(allowed_chars, i) == 0:
        res = False

# check condition
if res:
    print("True")

else:
    print("False")

Output

False

Time Complexity: O(N)

Auxiliary Space : O(1)

Method: Using the filter() function with a lambda function.

Algorithm:

1. Initialize the allowed_chars string with all the valid characters.
2. Initialize the string variable with the input string.
3. Use the filter() function with a lambda function to filter out any characters in string that are not in allowed_chars.
4. Convert the filtered object to a list and check if its length is equal to the length of the original string.
5. If the lengths are equal, output True. Otherwise, output False.

Python3

allowed_chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"

string = "__Val1d__"
filtered = filter(lambda c: c in allowed_chars, string)
print(len(list(filtered)) == len(string))
#this code contributed by tvsk

Output

True

Time complexity:
The time complexity of the filter() function is O(n), where n is the length of the input string. The time complexity of the list() function is also O(n). The time complexity of the comparison operation len(list(filtered)) == len(string) is O(1). Therefore, the overall time complexity of the code is O(n).

Auxiliary space:
The space complexity of the filtered list is O(n), where n is the length of the input string. The space complexity of the list() function is also O(n). Therefore, the overall auxiliary space complexity of the code is O(n).