WP User Manager - Custom field validation server side with regex

wpum_custom_field_regex_validation.php

<?php

add_filter( 'submit_wpum_form_validate_fields', function ( $check, $fields, $values, $form_name ) {
	foreach ( $fields as $group_key => $group_fields ) {
		$field_key = 'wpum_field_33'; // Change this to be your unique field ID with wpum_ prefix
    
    if ( ! isset( $values[ $group_key ][ $field_key ] ) ) {
			return $check;
		}

    $pattern = '/(a)(b)*(c)/'; // Change this to be your regex pattern
    
    preg_match( $pattern, $values[ $group_key ][ $field_key ], $matches);
		if ( empty( $matches ) ) {
			return new WP_Error( 'validation-error', __( 'This field does not meet the requirement', 'wp-user-manager' ) ); // Change error message
		}
	}

	return $check;
}, 10, 4 );

Save this file to your /wp-content/mu-plugins/ directory (you might need to create the mu-plugins directory).