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How to handle edge case while trying to grab all digits including decimal between two chars using regex

I am parsing a series of strings with various formats. The last edge case encountered has me stumped. I'm not a great regexer, believe me it was a challenge to get to this point.
Here are critical snippets from the strings I'm trying to parse. The second example is the current edge case I'm stuck on.
LBP824NW2-58.07789x43.0-207C72
LBP824WW1-77.6875 in. x 3.00 in. 24VDC
I am trying to grab all of the digits (including the decimal) that make up the width part of the dimension in the string (this would be the first number in the dimension). What works in every other case has been grabbing all digits from the "-" to the "x" using the following expression:
/-(\d+\.?\d+?)x\B/
However, this does not handle the cases that have inches included in the dimension. I thought about using "look-aheads" or "look-behinds", but I got confused. Any suggestions would be appreciated.

RegEx can be told to look for "zero or one" of things, using (...)? syntax, so if your pattern already works but it gets confused by a new pattern that simply has "more string data embedded in what is otherwise the same pattern" you can add in zero-or-one checks and you should be good to go.
In this case, putting something like (\s*in\.?\s*)? in a few tactical places to either match "any number of spaces (including none) followed by in followed by an optional full stop followed by any number of spaces (including none)" or nothing should work.
That said, "I cannot change the formatting" is almost never an argument, because while you can't change the formatting, you can almost always change what parses it. RegEx might be adequate, but some code that checks for what kind of general patter it is, and then calls the appropriate function for tokenizing and inspecting that specific string pattern should be quite possible. Unless you've been hired to literally update some predefined CLi script that has a grep in it and you're not allowed to touch anything except for the pattern...

This is the working solution using regex: -(\d+\.?\d+?)(\s*in\.?\s*|x)

Intellij IDEA regex character class may not be used inside character range

IDEA will not allow this error and i have not been able to find an option to turn off these kinds of errors. does anyone know how to fix the error or turn off the warning. The javascript works fine only IDEA sees this as a problem

You are creating a range by using the hyphen(-) in mid of your character class. You should move it to either end.
Also, note that you don't need to escape the regex meta-characters inside the character class. They loose their meanings in there.
So, just use:
[-\w._+%]

regex for background-image:url('URL');

I trying to make a regex for finding: background-image:url('URL'); Where the URL is a external link for an image.
Been trying for something like this:
/\s*?[ \t\n]background-image:url('https?:\/\/(?:[a-z\-]+\.)+[a-z]{2,6}(?:\/[^\/#?]+)+\.(?:jpe?g|gif|png)$');/i
But couldn't get it to work.
I am using this with javascript/jquery

Does this get what you want?:
/\s*?[ \t\n]background-image:url\('.+?'\);/i

I think you can simplify it to this if you know it will only change with the URL in the middle. I probably went overboard with the \ escapes but better to be safe than sorry.
/background\-image\:url\(\'.*?\'\)\;/

Epascarello hit the nail on the head. Is this source you control? Or at least a predictable website? What are multiple different examples of input and your expected results?
Will this always be inline in double quotes, and therefore your URL will always be in single quotes? Some old websites use double-quotes in their CSS Files or header CSS.
Do you want to capture the whole thing? Or are you just trying to extract the resulting URL?
SirCapsAlot brings up a good question, are you just looking for background image URL's in general? Because they can use the Background property also, or even be set in JavaScript with .backgroundImage="url(image.jpg)".
And you definitely only want the ones that include http(s)?
With the limited requirements you gave, this is the best Regex:
background-image\s*:\s*url\('(https?://[^']+)
Comment here if you have answers to my questions which may alter your requirements, and thusly my answer.
Breakdown:
background-image:\s*url //Find the literal text to begin
\(' //Find the literal opening parens and quote
( //Begin Capture Group 1
https?:// //Require the match of https:// (the s is optional because of the ?)
[^']+ //Require that everything until the next quote is matched
) //Capture the result into Group 1
A Co-Worker pointed out that I might have been downvoted for not capturing the closing tick. Note: Capturing the closing tick would be a wasted step, and is not necessary for this regex to work.
He also pointed out somebody might have downvoted me for requiring http or https in the url portion. But the user's question was specifically for external URLs, not internal ones. So this is a valid requirement and gets him closer to what he asked.
Sooo... not sure why this got a downvote.

Regex will not match

This is my string:
<link href="/post?page=4&tags=example" rel="last" title="Last Page">
From there I am trying to obtain the 4 out of that page parameter, using this regular expression:
link href="/post?page=(.*?)&tags=(.*?)" rel="last"
I will then collect the 4 out of the first group, the tags parameter has a wildcard because the contents can change. However, I don't seem to be getting a match with this, can anyone help?
And I know I shouldn't be using regex to parse HTML, but this is just a small thing and it would be a waste to import a huge module for this.

Assuming you are using a /regex literal/, you will need to escape the / in that path as \/.
Alternatively, it depends on how you are getting this string. Is it really typed that way, or is it part of an innerHTML that you are then reading out again? If that's the case, then the innerHTML won't be what you expect it to be, because the browser will "normalise" it.
If it is an innerHTML, then it'd be far easier to get the tag, then get the tag's href attribute, then regex that.

link href="/post\?page=(.*?)&tags=(.*?)" rel="last"
You forgot the slash before ?

I think it might be better to change your capture groups to something a little different, but will catch everything up to the terminating character:
link href="/post?page=([^&]+)&tags=([^\"]+)" rel="last"
Using the negating character first in the character group tells the regex engine "capture all characters EXCEPT the ones listed here". This makes it very easy to capture everything up until it hits a termination character, such as the amperstand and double-quote. Assuming you're using PHP or Java, this should also slightly improve regex performance.

If the page parameter always comes first, try the PCRE /\?page=(\d+)/. Match group 1 will contain the page number.

Stuck on a regular expression

I seem to have backed myself up into a corner with this. I'm sure that the answer is going to make me want to smack a brick against my head, but I'm not all that good with regex just yet. So, here goes.
I need to modify this regex so that it fails if it finds any occurances of pound signs. (#)
My current regex is this;
/^[A-Za-z\.\-\_\s]{1,80}$/i
I tried a number of variations such as;
/[^#]^[A-Za-z\.\-\_\s]{1,80}$/i
/^[[^#]A-Za-z\.\-\_\s]{1,80}$/i
/^[A-Za-z\.\-\_\s^#]{1,80}$/i
None of which work. Can anyone offer any advise, please?

Your original regex should work, because # isn't in the list of characters you specified for the class. You don't need to add anything, it already fails if there's a # in there.

Just use two regexes:
/^[A-Za-z\.\-\_\s]{1,80}$/i
Then filter your input so that you keep only what does NOT match this regex:
/#/
It's far easier to match on patterns that you want to filter out (and then ignoring the matching strings instead of ignoring the complement) than it is to try to construct an "inverse" regex. And there's no reason why you should try to fit everything into one regex.