How to Check if there are Numbers in Strings in Python

Determining if a string contains numeric characters is a common task in Python.

This guide demonstrates various methods for checking if a string contains numbers, whether it contains only numbers, including techniques using generator expressions, regular expressions, string methods and built-in functions.

Checking if a String Contains a Number

Several approaches can be used to determine if a string contains any digits.

Using a Generator Expression and any()

This method combines a generator expression with the any() function for a concise solution:

def contains_number(string):
    return any(char.isdigit() for char in string)

print(contains_number('abc123')) # Output: True
print(contains_number('abc'))    # Output: False
print(contains_number('-1abc'))  # Output: True

• The isdigit() method checks if all characters in a string are digits. Returns True if the string contains only digits and is not empty, otherwise returns False.

Using a for Loop

A for loop provides a more explicit approach:

def contains_number_loop(string):
    for char in string:
        if char.isdigit():
            return True  # Return True immediately if digit found
    return False  # Return False if no digits found after loop

print(contains_number_loop('abc123'))  # Output: True
print(contains_number_loop('abc'))     # Output: False
print(contains_number_loop('-1abc'))   # Output: True

• The function iterates over the characters and immediately returns True if a digit is encountered.
• If no digits are present it returns False.

Using re.search()

The re.search() method, which uses regular expressions, provides a more flexible approach that allows for looking for more complex patterns, and can also match multiple different digits at once:

import re

def contains_number_regex(string):
    return bool(re.search(r'\d', string))

print(contains_number_regex('abc123')) # Output: True
print(contains_number_regex('abc'))    # Output: False
print(contains_number_regex('-1abc'))  # Output: True

• re.search(r'\d', string) returns a match object if a digit is found, and None if not found, which then bool() casts to True or False respectively.

Using re.findall()

The re.findall() method finds all occurrences of the pattern and returns a list of matches. This method is especially useful when there are multiple matches:

import re
my_str = 'a12b34c56'
matches = re.findall(r'\d+', my_str)      # r'\d+' matches one or more consecutive digits.
print(matches)                            # Output: ['12', '34', '56']
if len(matches) > 0:
    print('The string contains a number') # Output: The string contains a number
else:
    print('The string does NOT contain a number')

Checking if a String Contains Only Numbers

To ensure a string only contains numbers, use the isnumeric() method, try/except or a regular expression.

Using isnumeric()

The isnumeric() method is a direct way to check for numeric characters, including digits and other Unicode numeric characters. It does not, however, include characters like - or ..

my_str = '3468910'
print(my_str.isnumeric()) # Output: True

if my_str.isnumeric():
    print('The string contains only numbers') # Output: The string contains only numbers
else:
    print('The string does NOT contain only numbers')


print('5'.isnumeric())      # Output: True
print('-50'.isnumeric())    # Output: False
print('3.14'.isnumeric())   # Output: False

Validating Integers and Floats with try/except

If you need to determine that the string is a valid number use a try/except block to handle potential ValueError during casting.

def is_integer(string):
    try:
        int(string)
    except ValueError:
        return False
    return True

print(is_integer('359'))   # Output: True
print(is_integer('-359'))  # Output: True
print(is_integer('3.59'))  # Output: False


def is_float(string):
    try:
        float(string)
    except ValueError:
        return False
    return True


print(is_float('359'))  # Output: True
print(is_float('-359')) # Output: True
print(is_float('3.59')) # Output: True

• The functions attempt to cast the string using either int() or float().
• The try/except block gracefully handles invalid strings, preventing errors and returning False in such cases.

Using re.match()

A more direct way to validate that a string consists only of digits is using regular expressions:

import re

def contains_only_numbers(string):
    return bool(re.match(r'^[0-9]+$', string))

print(contains_only_numbers('3590'))  # Output: True
print(contains_only_numbers('3x59'))  # Output: False

• The regex pattern ^[0-9]+$ matches a string containing one or more digits from start (^) to end ($) of the string, effectively checking that it contains only numbers.