Bitcoin Address (Legacy P2PKH — starts with 1) Regex for PHP

What this pattern does

This page provides a lightweight, single-purpose regular expression for matching bitcoin address (legacy p2pkh — starts with 1), ported and verified for PHP. Financial data validation has zero tolerance for false negatives — a missed invalid entry can corrupt downstream calculations. The snippet below is ready to drop into your PHP project — whether you're validating in a Laravel validator, a WordPress plugin, or a standalone PHP script.

Php Implementation

<?php
// Bitcoin Address (Legacy P2PKH — starts with 1)
// ReDoS-safe | RegexVault — Finance > Crypto

define('BITCOIN_ADDRESS_LEGACY_P2PKH_STARTS_WITH_1_PATTERN', '/^1[a-km-zA-NP-Z1-9]{25,34}$/');

function validate_bitcoin_address_legacy_p2pkh_starts_with_1(string $input): bool {
    return (bool) preg_match(BITCOIN_ADDRESS_LEGACY_P2PKH_STARTS_WITH_1_PATTERN, $input);
}

// Example
var_dump(validate_bitcoin_address_legacy_p2pkh_starts_with_1("1A1zP1eP5QGefi2DMPTfTL5SLmv7Divf")); // bool(true)

Test Cases

When to use this pattern

This pattern is drawn from the Finance > Crypto category and carries a ReDoS-safe certification. That matters for PHP developers because especially relevant in PHP where PCRE backtracking limits can trigger silent failures on malicious input. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

P2PKH addresses start with 1. Base58Check encoding (excludes 0, O, I, l to avoid confusion). Length is 25-34 chars. The address encodes a hash of the public key — validate the checksum with actual Base58Check decoding.