PHP regex check length without using lookahead - php

Question

Looked at 2 other regex related questions and both were vastly beyond what I need help with X'D.
<?php
$userName = trim($_POST['username']);
if(preg_match("/^[a-zA-Z].*(?(\d)).{8,}$/", $userName)){
echo 'woot!';
I'm not really sure why its failing. I know its checking the first character must be alpha, then go more * but check at the end if its a digit ?(\d)... But why wont it check if the length of my $userName is 8 or more?
if I do
... *(?(\d){8,}
Its going to look for 8 or more digits as opposed to if the string itself is 8 or more.
User must: start with letter, only end in numbers, and can only contain alphaNumeric, must be at least characters 8 long, and treat lower and uppercase the same. Which is to say I need to add an i at the end of my regex.
As so there is no way to work it into a regex aside form look aheads? ^...$.{8,} wouldn't work?

Answer 1

User must: start with letter, only end in numbers, and can only contain alphaNumeric, must be at least characters 8 long, and treat lower and uppercase the same.
You may use this simple regex to meet this requirement:
^[A-Za-z][a-zA-Z\d]{6,}\d$
RegEx Details:
^: Start
[A-Za-z]: Match an ASCII letter at the start
[a-zA-Z\d]{6,}: Match a letter or digit 6 or more times
\d: Match a digit before end
$: end
RegEx Demo

Answer 2

Use the following regex pattern:
^(?=.{8,}$)[A-Z][A-Z0-9]*[0-9]+$
Sample script:
$userName = trim($_POST['username']);
if (preg_match("/^(?=.{8,}$)[A-Z][A-Z0-9]*[0-9]+$/i", $userName)) {
echo "MATCH";
}
Of note, the regex pattern uses (?=.{8,}$) at the start of the pattern. This is a positive lookahead, which asserts that the match is at least 8 characters.
If you don't want to use a lookahead, you may just explicitly check the username length using strlen():
$userName = trim($_POST['username']);
if (preg_match("/^[A-Z][A-Z0-9]*[0-9]+$/i", $userName) &&
strlen($userName) >= 8) {
echo "MATCH";
}