[match <? and replace with <?php]

$result = preg_replace('/<\?(?!php)/', '<?php', $subject);

[replace "-" with "/" in preg_match pattern]

Just change the two / to be ~ and then change the two )-( to read )/(

[BBCode Issue]

change

[/b]

to

[\/b]

[I just need help... :(]

ha yeah I like to keep my groups just so I know where everything is tbh, you are right though it was unnecessary. Love the one liner!

[I just need help... :(]

<?php

$str = "R1,R2-5,R7";
$parts = explode(',', $str);
$out = array();
$regex = '~^([^\d]+)(\d+)-([^\d]*)(\d+)$~';

foreach($parts as $part) {
if(preg_match($regex, $part, $matches)) {
	$range = range($matches[2], $matches[4]);
	foreach($range as $number) {
		$out[] = $matches[1] . $number;
	}
} else {
	$out[] = $part;
}
}

echo '<pre>' . print_r($out, true) . '</pre>';

?>

 

That's how I would do it. Should work for your last example too

[I just need help... :(]

[^\d]+ will match everything before the numbers

[I just need help... :(]

You aren't going to be solving this using regex, at least not all of it. You're going to need to explode the values using the , then us a regex to match the 2-5, then use a for loop to iterate over the values and add them correctly to the array

[Escaping strings for GLOB]

are you wanting the addslashes() function?

[replace everything except...]

You can just put the forward slash since you're using # as your delimiter

[Search for keyword in sentence]

$abc = "I drink water. I drink vodka too.";
$regex = '~\b(water|vodka)\b~i';

if(preg_match($regex, $abc)) {
    // Found a match
}

[Need a little regex help. My first little scrape.]

I only need one little piece of data from this page. And the data that I want is in a span tag with an id.. Like so....

 

<span id="TotalDue">$1,004.28</span>

I'm pretty sure from this quote that its part of a full page of text

[Need a little regex help. My first little scrape.]

Surely that would match the data between all tags on a page, not just the specific data in the span tag that the OP requested?

I'm assuming the string given is only part of the page, not the whole thing

[Need a little regex help. My first little scrape.]

Note that in my regex I used ~ ~ as the start/end delimiters, instead of /

That is why you are getting an error. While using the DOM Document is great, it's probably overkill for your example

[Need a little regex help. My first little scrape.]

~<span id="TotalDue">([^<]+)</span>~

 

The value will be stored in capture group 1

[Creepy Warning]

I was being sarcastic  It's not me that needs to know how to fix the error lol

[Creepy Warning]

... but that involves reading ...

[Creepy Warning]

This isn't related to regex at all!

This is because you're outputting content before your cookies are set, which you can't do. There must be a million articles on why this error happens 

[Strip non alphanumeric from start of string]

haha I know that feeling

[Strip non alphanumeric from start of string]

$output = preg_replace('~^[^\da-z]+~i', '', $input);

 

That will replace any characters at the start of a string up until the first letter or number only

[Remove everything from a string after the third slash]

You would be better off using parse_url

<?php
$text = 'http://www.somedomain.com/blah/blah/blah';
$parts = parse_url($text);
echo $parts['scheme'] . '://' . $parts['host'] . '/';

 

For the regex version, this will do it

<?php
$text = 'http://www.somedomain.com/blah/blah/blah';

$result = preg_replace('~^(.*?//.*?/).*$~', '$1', $text);
echo $result;

[I hate preg_match!]

If you're wanting to restrict it to just text either side as the "tags" you could use

function getTextBetweenTags($string, $start, $end) {
$start = preg_quote($start, '~');
$end = preg_quote($end, '~');
$pattern = "~$start(.*?)$end~";

if(preg_match($pattern, $string, $matches)) {
	return $matches[1];
}

return false;
}

$html = 'this is a bunch of nothing _MARKER_OBJ = [ get this crap ]';

$content = getTextBetweenTags($html, '_MARKER_OBJ = [', ']');

echo $content;

 

That doesn't require you to escape the start/end tags as it escapes all regex syntax characters using preg_quote

[Over 18 regex]

I want a person to enter their age but it has to be 18 or over and up to 99

This is why my solution stops at 99 

[Over 18 regex]

Ah OK, I skipped that post sorry. my regex above should work

[Over 18 regex]

Also, if you really want to do it via regex...you could use this, although personally I find it a bit overkill

~^(1[89]|[2-9][0-9])$~

[Over 18 regex]

Why would you use regex for this?

Surely it's easier to just use regular php

 

$age = (int) $_POST['age'];
if($age >= 18 && $age <= 99) {
    // Age is correct
} else {
    // Age is incorrect
}