How to make a fixed left menu?

Question

Regular Expressions

1) Good afternoon everyone. Help a newbie, please, I can't really find a solution. Sorry for such a picchu, I drew it on the way. The problem is that I need to make a fixed left menu, and at the same time make the site to be placed in the center of the screen. If you set the menu block to position: fixed, then it drops out of the general flow (the block is filled with red), and I need it to be in the general block. If done in a common block, it will scroll along with everything else. I want to make up on bootstrap 4. Tell me how it is generally better to implement such markup.
2) I’m completely insolent, maybe there is someone here who would agree to mentor me a little in layout and JS. I don’t need much, sometimes answering such stupid questions, sometimes just pointing out what to do. Ideally, if Kyiv, I can thank you with some other services, such as go and pick up or something like that. Thanks in advance, sorry if it's a bit off topic.

Answer 1

So?
https://regex101.com/r/18PVEe/2
Or like this:
https://regex101.com/r/18PVEe/3/
In both cases, I assume that there are no characters in either the error line or the message line !#.

Answer 2

(?:.*#!(.*)!#)|(?:.*#(.*)!!#)
first capture group error
second capture group message
like check that there is no error first
message must not start with an exclamation mark sign and contain a hash (the hash just substrings the contents), but it can end with an exclamatory porridge
Wddd!!! s!!!
Wdd!!! s!!v!
and an error cannot contain #! (just substring's content) can have an exclamatory mess anywhere
!!!Wddd!!!
!s!s!Wddd!s!!
#0234 !s!Wddd!s!!
!s!##W#ddd!#s only at the end of the reshotka can't stand
probably everything

Answer 3

Just set MAIN's margin-left to the width of the red box.
It can be physically in CSS, or it can be dynamic in JS or jQuery, it doesn't matter.
Just a sketch https://jsfiddle.net/webirus/9vg3pud0/1/