How do I verify that a string only contains letters, numbers, underscores and dashes in Python?

To verify that a string contains only letters, numbers, underscores, and dashes in Python -

• Use re.fullmatch() function with a pattern like [A-Za-z0-9_-]+ for regex-based checking.
• Use set comparison with all() function for simple logic-based validation.

Many systems restrict input to certain characters for security or formatting reasons. In this case, the allowed characters are alphabets (A-Z, a-z), digits (0-9), underscores (_), and hyphens (-).

Using Regular Expressions

The re module in Python allows you to define patterns to validate strings. You can use re.fullmatch() function to check if the entire string matches a specific pattern, such as allowing only letters, digits, underscores, or dashes.

Example

In the following example, we use re.fullmatch() function with a Regex pattern that matches the entire string -

import re

def is_valid(text):
   return re.fullmatch(r"[A-Za-z0-9_-]+", text) is not None

print(is_valid("User_123-Name"))   # Valid
print(is_valid("Invalid@Name"))    # Invalid

Here, the pattern [A-Za-z0-9_-]+ ensures that all characters belong to the allowed set -

True
False

Using Set Comparison

Set comparison checks if all characters in the string are part of a predefined set of valid characters like letters, numbers, underscores, or dashes. It is a simple way to validate custom rules without regular expressions.

Example

In this example, we compare characters with a predefined set -

def is_valid(text):
   allowed = set("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_-")
   return all(char in allowed for char in text)

print(is_valid("Safe_String-42"))  # Valid
print(is_valid("Oops!"))           # Invalid

This method gives you full control over character rules and works without importing external modules -

True
False