PHP regex check length without using lookahead

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?

Please clearly tell us what the rules are for a username? It is clear that it must start with a letter, and also have a digit at some later point. The rest is not clear. — Tim Biegeleisen, May 30 '21 at 05:01
User must: start with letter, only end in numbers, and can only contain alphaNumeric, must be atleast 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. — Coding_Noob, May 30 '21 at 05:07

score 3 · Answer 1 · answered May 30 '21 at 07:46

3

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

answered May 30 '21 at 07:46

anubhava

761,203
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1

You could simplify this to `^[A-Z][A-Z\d]{6,}\d$` and use it in case insensitive mode. – Tim Biegeleisen May 30 '21 at 12:36
Yes true we could use `/i` like that. I guess I just wanted to give complete solution in regex only for readability purpose otherwise `/^[A-Z][A-Z\d]{6,}\d$/gmi` is shorter. – anubhava May 30 '21 at 13:12
@Coding_Noob: Did this solution work for you? – anubhava Jul 05 '21 at 06:17

Tim Biegeleisen · Answer 2 · 2021-05-30T05:11:40.843

0

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";
}

edited May 30 '21 at 05:11

answered May 30 '21 at 05:10

Tim Biegeleisen

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Ye I saw a few things regarding look aheads; is there any way to do this without? We havent covered that in the course I'm in, and I'd really like to understand why its not working as is with my regex; or how it could work :) ? – Coding_Noob May 30 '21 at 05:11
Use `strlen()` to check the number of characters? – Tim Biegeleisen May 30 '21 at 05:13
As so theres no way to work it into a regex aside form look aheads? ^...$.{8,} wouldn't work? – Coding_Noob May 30 '21 at 05:20
1

(You could omit the last `+` after `[0-9]+` as the character class before it also matches digits ) – The fourth bird Jun 18 '21 at 16:25

PHP regex check length without using lookahead

2 Answers2