Validate IBANs, credit cards, and emails in your browser
Every online form that collects payment or contact details needs validation, but sending sensitive data to a server just to check formatting opens your information to interception and storage. Client-side validation using checksums and pattern matching catches errors before submission and keeps your data local. Understand what these validation methods can and cannot prove—and why running them in your browser matters for privacy and accuracy.
Why format validation matters
Format validation is your first line of defense against typos and honest mistakes. A single transposed digit in an IBAN or credit card number makes the entire account number invalid and the transaction will fail. Catching these mistakes before submission saves time, prevents failed payments, and protects against frustrating error messages on the server side. Format validation also serves as an early warning system: if a number fails a basic checksum test, you know to double-check before sending it anywhere.
Beyond catching typos, format validation protects your privacy. If you can spot an error locally before hitting send, you reduce the surface for accidental data transmission. A validation tool running in your browser never needs to store, log, or transmit what you have entered.
What checksum validation can and cannot prove
Checksum algorithms like Luhn and mod-97 verify that a number is internally consistent—that its digits follow a mathematical pattern. A valid checksum proves the number was typed correctly and belongs to the right format family. It proves nothing about whether the account exists, whether it is active, whether the card has funds, or whether the person entering the data owns it.
Think of checksums as consistency checks, not identity verification. A valid credit card number could be entirely fictional and still pass the Luhn test. A correctly formatted IBAN that has not been used in five years will still validate perfectly. Checksums catch typos and format errors; they do not perform account lookups or access financial systems. For real validation, only a server talking directly to a bank or payment processor can confirm that an account is live and authorized for a transaction.
Validating IBANs with mod-97
International Bank Account Numbers (IBANs) standardize how bank account details are written across different countries. Each country has an IBAN with a fixed length—Germany is always 22 characters, Spain always 24—and a specific structure: country code, check digits, and the account identifier. The check digits use mod-97 arithmetic to encode a mathematical checksum.
The validation process is elegant: rearrange the IBAN by moving the country code and check digits to the end, convert each letter to its position in the alphabet plus nine (A=10, B=11, and so on), then compute the entire number modulo 97. If the result equals 1, the IBAN is structurally valid. The IBAN validator performs this calculation in your browser instantly, catching transposed digits, missing characters, and invalid country codes without contacting any bank.
Credit card validation with Luhn
Credit and debit cards use the Luhn algorithm, a lightweight checksum developed in 1954 that remains the industry standard. The algorithm works by doubling every second digit starting from the right, subtracting 9 from any result larger than 9, then summing all digits. If the total is divisible by 10, the card number passes the Luhn check.
This simple math catches common mistakes: a single digit transposed, a digit skipped during entry, a digit duplicated or deleted. The credit card validator runs the Luhn test in milliseconds without storing or transmitting your card details. A valid Luhn sum proves your card number follows the correct structure for Visa, Mastercard, American Express, or other issuers. It does not prove the card is active, unfraudulently obtained, has available balance, or belongs to you.
Email and URL format validation
Email and URL validation is simpler than checksums but equally important for catching human error. An email address must contain an @ symbol with a local part before it and a domain with a valid top-level domain (.com, .org, .co.uk, and so on) after it. A URL must begin with a valid scheme (http://, https://, ftp://) followed by a domain and optional path.
The email validator and URL validator parse these formats and reject obvious mistakes: missing @, spaces or special characters in illegal positions, missing top-level domains, typos in common domains, malformed schemes. Like checksums, format validation here is pattern-matching against known good structures. It proves the syntax is correct; it does not prove the email address has an active mailbox or that the URL will respond.
Privacy through client-side validation
Every validation tool on TextArray runs entirely in your browser. When you paste a card number, IBAN, email, or URL, it never touches a server. There is no account to create, no password to manage, no cookies to store. The tools continue working after the page loads, even if your network connection drops offline.
This architecture protects your sensitive data by design. Your input remains on your device, visible only to you. You get immediate feedback—is this number formatted correctly?—without any third party receiving, storing, or processing what you have entered. That is the whole point of client-side validation: check for errors where the data lives, before deciding whether to send it anywhere.
Validation as one layer in a secure flow
Client-side checksum and format validation are the first line of defense, catching honest typos and user errors. They are not a complete security solution. Real validation—confirming that an IBAN belongs to an active account, a credit card is authorized and has funds, or an email address will accept mail—happens on servers controlled by the actual financial institution or service provider.
Use these tools to validate format before submission. Trust server-side systems to verify that accounts and cards are real. By catching errors locally first, you streamline your transaction flow and reduce the volume of malformed requests that servers must reject. Together, client-side format checking and server-side account verification create a reliable, privacy-respecting validation pipeline.