Python Program to check if String contains only Defined Characters using Regex

When it is required to check if a given string contains only specific characters using regular expression, a regular expression pattern is defined, and the string is subjected to follow this pattern.

Understanding the Regex Pattern

The pattern ^[Python]+$ breaks down as follows:

• ^ − Start of string
• [Python] − Character set containing P, y, t, h, o, n
• + − One or more occurrences
• $ − End of string

Example

Below is a demonstration of the same −

import re

def check_string(my_string, regex_pattern):
    if re.search(regex_pattern, my_string):
        print("The string contains the defined characters only")
    else:
        print("The string does not contain the defined characters only")

# Compile regex pattern - only letters P, y, t, h, o, n allowed
regex_pattern = re.compile('^[Python]+$')

my_string_1 = 'Python'
print("The string is :")
print(my_string_1)
check_string(my_string_1, regex_pattern)

my_string_2 = 'PythonInterpreter'
print("\nThe string is :")
print(my_string_2)
check_string(my_string_2, regex_pattern)

# Additional test cases
my_string_3 = 'Pynot'
print("\nThe string is :")
print(my_string_3)
check_string(my_string_3, regex_pattern)

my_string_4 = 'hello'
print("\nThe string is :")
print(my_string_4)
check_string(my_string_4, regex_pattern)

Output

The string is :
Python
The string contains the defined characters only

The string is :
PythonInterpreter
The string does not contain the defined characters only

The string is :
Pynot
The string contains the defined characters only

The string is :
hello
The string does not contain the defined characters only

Alternative Approach Using match()

You can also use re.match() which checks if the pattern matches from the beginning of the string −

import re

def validate_string(text, allowed_chars):
    pattern = f'^[{allowed_chars}]+$'
    return bool(re.match(pattern, text))

# Test with different character sets
test_string = "Python"
allowed_chars = "Python"

result = validate_string(test_string, allowed_chars)
print(f"String '{test_string}' contains only '{allowed_chars}': {result}")

# Test with numbers
test_string2 = "12345"
allowed_chars2 = "12345"

result2 = validate_string(test_string2, allowed_chars2)
print(f"String '{test_string2}' contains only '{allowed_chars2}': {result2}")

String 'Python' contains only 'Python': True
String '12345' contains only '12345': True

How It Works

• The re module is imported to work with regular expressions
• A function check_string takes the string and compiled regex pattern as parameters
• The re.search() method checks if the pattern matches anywhere in the string
• The pattern ^[Python]+$ ensures the entire string contains only the characters P, y, t, h, o, n
• If the pattern matches, the string contains only defined characters; otherwise, it contains other characters

Conclusion

Regular expressions provide a powerful way to validate if strings contain only specific characters. Use ^[characters]+$ pattern to ensure the entire string matches the allowed character set.