CRON time expression validation with regex

CRON time expression validation with regex.md

CRON time expression validation with regex

If you're looking for a regex expression which can validate the format of a CRON time expression (for most versions of CRON) you've probably had a look at StackOverflow and had to trapse through a bunch of possible expressions which may not quite work.

Credit to Juarez and Slava who collectively derived the expression below here: https://stackoverflow.com/questions/14203122/create-a-regular-expression-for-cron-statement/63729682#63729682

^((((\d+,)+\d+|(\d+(\/|-|#)\d+)|\d+L?|\*(\/\d+)?|L(-\d+)?|\?|[A-Z]{3}(-[A-Z]{3})?) ?){5,7})|(@(annually|yearly|monthly|weekly|daily|hourly|reboot))|(@every (\d+(ns|us|µs|ms|s|m|h))+)$

Warning

This is intended to validate ONLY the time expression, not a line from a CRON file.

If you're interested in what this looks like in use, have a look at: https://regex101.com/r/RtLgqG/

If you use this, take care to check which version of CRON expressions are supported and amend this regex as needed, i.e. removing specific parts between the OR "|" pipe symbols.

e.g. if your version of CRON does not support the @TIME expressions:

^((((\d+,)+\d+|(\d+(\/|-|#)\d+)|\d+L?|\*(\/\d+)?|L(-\d+)?|\?|[A-Z]{3}(-[A-Z]{3})?) ?){5,7})$

Examples of use

These examples use the full expression.

Bash / Shell function

#!/bin/bash

validate_cron_expression() {
    local cron_expression="$1"
    local regex='^((((\d+,)+\d+|(\d+(\/|-|#)\d+)|\d+L?|\*(\/\d+)?|L(-\d+)?|\?|[A-Z]{3}(-[A-Z]{3})?) ?){5,7})|(@(annually|yearly|monthly|weekly|daily|hourly|reboot))|(@every (\d+(ns|us|µs|ms|s|m|h))+)$'

    if echo "$cron_expression" | grep -Pq "$regex"; then
        return 0  # Valid cron expression
    else
        return 1  # Invalid cron expression
    fi
}

# Example usage:
cron_expression="* * * * *"
if validate_cron_expression "$cron_expression"; then
    echo "Valid cron expression: $cron_expression"
else
    echo "Invalid cron expression: $cron_expression"
fi

Note

In the context of bash scripting:

• If a command or function completes successfully without any errors, it typically returns an exit status of 0.
• If there is an error or the command/function encounters an issue, it returns a non-zero exit status to indicate failure.

When we check the return value of the validate_cron_expression function using the if statement, we're checking if the function returned true (0) or false (non-zero).

Python function

#!/usr/bin/env python3

import re

def validate_cron_expression(cron_expression):
    regex = '^((((\d+,)+\d+|(\d+(\/|-|#)\d+)|\d+L?|\*(\/\d+)?|L(-\d+)?|\?|[A-Z]{3}(-[A-Z]{3})?) ?){5,7})|(@(annually|yearly|monthly|weekly|daily|hourly|reboot))|(@every (\d+(ns|us|µs|ms|s|m|h))+)$'

    if re.match(regex, cron_expression):
        return True  # Valid cron expression
    else:
        return False  # Invalid cron expression

# Example usage:
cron_expression = "* * * * *"
if validate_cron_expression(cron_expression):
    print(f"Valid cron expression: {cron_expression}")
else:
    print(f"Invalid cron expression: {cron_expression}")