How to validate a form field against a regular expression in HTML5?
Sep 22, 2025 am 05:11 AM
Use the pattern attribute in HTML5 input elements to validate against a regex, such as for passwords requiring numbers, uppercase, lowercase, and minimum length; pair with title for user guidance and required for non-empty enforcement.
To validate a form field against a regular expression in HTML5, use the pattern attribute within an input element. This attribute accepts a JavaScript-style regular expression that the input value must match to be considered valid.
Using the pattern Attribute
The pattern attribute works on input fields of type text, tel, email, password, and others where custom text formatting is needed. When the form is submitted, the browser checks if the input matches the specified regex.
- Include the title attribute to provide a helpful message explaining the expected format
- The validation runs automatically during form submission or when using constraint validation APIs
- No JavaScript is required for basic validation
Example: Validate a Password Format
Suppose you want a password to contain at least one number, one lowercase letter, one uppercase letter, and be at least 8 characters long:
The browser will display the title text if the input doesn't match the pattern.
Limitations and Notes
The pattern attribute does not apply when the input field is empty. If you need to enforce input, combine it with the required attribute.
- Browser support is good across modern browsers, but older versions may not support it
- Always validate on the server side as well—client-side validation can be bypassed
- Complex patterns might be harder to debug since error messages are limited to the title text
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