pewc_filter_validate_cart_item_status.php

<?php
/**
 * Validate a text string to check first character is a letter and 2nd to 7th characters are numeric
 */
function prefix_filter_validate_text_string( $passed, $post, $item ) {

  // Change the field ID as necessary
  if( isset( $item['field_id'] ) && $item['field_id'] == 4904 ) {
		$string = $post[$item['id']];
		$first_letter = substr( $string, 0, 1 );
		$six_letters = substr( $string, 1, 6 );
		if( ! ctype_alpha( $first_letter ) ) {
			$passed = false;
			wc_add_notice( apply_filters( 'pewc_filter_validation_notice', esc_html( $item['field_label'] ) . __( ' must have a letter as the first character.', 'pewc' ), $item['field_label'], $item ), 'error' );
		}
		if( ! ctype_digit( $six_letters ) ) {
			$passed = false;
			wc_add_notice( apply_filters( 'pewc_filter_validation_notice', esc_html( $item['field_label'] ) . __( ' must have numbers as the second to seventh characters.', 'pewc' ), $item['field_label'], $item ), 'error' );
		}
  }
  return $passed;
}
add_filter( 'pewc_filter_validate_cart_item_status', 'prefix_filter_validate_text_string', 10, 3 );