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stackbiz
Asked: 2024-10-20 17:15:09 +0800 CST

How to check allowed characters in bash with regex

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I want to use regex for checking only three allowed characters: "[0-9]" and "." and "/".

Note: The test case is an ipv4/ipv6 address but I don't want to check the number in the range [0-255], I only want to check allowed characters in the input variable.

I use "|" as the "or" expression, and combine all the three allowed characters with "|", and finally add "+" in the last for make sure at less one allowed character must exist in the input variable.

Here is the test bash script:

test_ipv4_address="127.0.0.1/24"
test_wrong_ipv4_address="127.0.0.1#24"
test_ipv6_address="::1/128"

allowed_characters='([0-9]|\.|\/)+'

[[ "$test_ipv4_address" =~ $allowed_characters ]] && echo "yes, $test_ipv4_address is allowed"
[[ "$test_wrong_ipv4_address" =~ $allowed_characters ]] && echo "yes, $test_wrong_ipv4_address is allowed"
[[ "$test_ipv6_address" =~ $allowed_characters ]] && echo "yes, $test_ipv6_address is allowed"

Here is the output:

yes, 127.0.0.1/24 is allowed
yes, 127.0.0.1#24 is allowed
yes, ::1/128 is allowed

Here is the expected output:

yes, 127.0.0.1/24 is allowed

What's wrong in my regex? And how to fix?

command-line

1 Answers

  1. Best Answer

    Your regular expression seems almost correct, but it could use some slight adjustments:

    1. The . character in regex has a special meaning (matching any character), so it needs to be escaped using \. to match literal dots.
    2. You're already escaping . properly, but the key issue is how you're handling regex within bash. Bash treats regex differently than other languages, so double-check how it's applied.

    Now the modified script will look something like this:

    test_ipv4_address="127.0.0.1/24"
test_wrong_ipv4_address="127.0.0.1#24"
test_ipv6_address="::1/128"

# Regex to allow only digits, dots, and slashes
allowed_characters='^[0-9./]+$'

# Check if the input matches the allowed characters
[[ "$test_ipv4_address" =~ $allowed_characters ]] && echo "yes, $test_ipv4_address is allowed"
[[ "$test_wrong_ipv4_address" =~ $allowed_characters ]] && echo "yes, $test_wrong_ipv4_address is allowed"
[[ "$test_ipv6_address" =~ $allowed_characters ]] && echo "yes, $test_ipv6_address is allowed"

    And you will get an desired output like this:

    └─$ ./test.sh 
yes, 127.0.0.1/24 is allowed
    • 3