Can anyone give me a regex that I can use on $username to limit characters to [A-Z][a-z][0-9][@.-_] (including not allowing white space)?

What is "to limit" ? Regular expressions either match or don't.

...

Anyway, have you read 'man perlretut' ? Specifically, "Using character classes" ? It's the exact chapter you have to read.

When I say limit, I was thinking:

Remove all non-[A-Za-z0-9@.-_] characters

What I REALLY want is:

if ($username != [A-Za-z0-9@.-_]){

print "unallowed characters\n";
}

But one that works...

Have you ever looked at regular expression matching ?

I.e. 'perldoc perlop', then look for

m/PATTERN/cgimosx
/PATTERN/cgimosx
.

You've made a number of trivial mistakes in your code (I think four), and the suggested man pages have the answers.

I looked at those pages but, I don't understand Greek. So, let me re-ask...

Can anyone give me a regex that will match non-[A-Za-z0-9@.-_] characters in an IF statement?

It's not Greek, it's English.

The answer to your question, and the fix to you most severe mistake, is is looking again into

'perldoc perlop', then looking for

m/PATTERN/cgimosx
/PATTERN/cgimosx

, then paying attention to =~ characters.

Regular expressions are a computer language - like Perl or "C" or Python or Java.

When you are saying "I don't understand Greek", you are actually saying "I do not want to study a language, bring me a solution".

What is required from you is to read and comprehend from in my case line #712 until line #765, i.e. 54 lines of English. I mean line numbers in

perldoc perlop

screen output.

By the way, if after reading line numbers 712 .. 765 you still don't understand something, post here particular questions.

If you do not specifically explain what you do not understand, we do not know what exactly to explain.

If you want to play with regexes, you're going to need to learn a little Greek. You might have better luck if you start with perldoc perlrequick. In any case, here's a kind of ham-handed approach:
In a nutshell [] creates a character class. It says look for the stuff inside the square brackets. If you use [^], you are doing a negative character class. So it says, look for anything other than what's inside the square brackets. So, I took what you said were the allowed characters, and I said "If you find anything that isn't one of these, scold the user." The only real trick there is that if you want to search for a literal '-' in a character class you need to put it first or escape it (\-). Other than that, it's pretty much straightforward.

Sergei, thank you for your replies but, I am not a programmer and I do not have that mindset. I try and do as little coding as possible.

'RTFM' replies do me no good. I need examples of how things work. I have read Perl docs until my eyes bleed and I do not understand them (same with man pages), they are useless to me. I need examples to understand.

Telemachos reply, "/[^-A-Za-z0-9@._]/" explains it all to me...

// = This is a regex
^ = not the following
-A-Za-z0-9@._ = allowed characters
[] = match the characters inside these.

I understand.

54 lines of Perldoc gibberish just confuses me. It is the same as Greek.

I would like to build a house. I don't want to use a hammer or a saw, and I don't want to learn any builder's techniques.
--- rod.

theNbomr I would like to build a house. I don't want to use a hammer or a saw, and I don't want to learn any builder's techniques.
--- rod.

No, more like: "You want to build a house? Here's a book (with no pictures), figure out how to do dovetail joints by reading 54 lines of text."

If I wanted elitism, I'd post on FreeBSD forums...

After having read quite a bit about construction methods, I do know that dovetail joints are rarely used in homebuilding. And the point is that if you want to do something that you don't already know, it serves you well to do a little studying. That's how those who you expect to hand you an answer learned it.

Allegedly, you want advice on programming, since 'This forum is for all programming questions' and you put a programming language in the subject line of your post. It isn't elitism, it is on-topic. Specific references to authorative documents seem like appropriate forms of help. The helpful people in this forum are generally loath to simply do your work for you, but are almost universally willing to help you learn for yourself.

--- rod.

@joe. why don't you do your best?

theNbomr - I don't know of one single carpenter that ever learned how to build a house by reading a manual. They all learned by example (watching it being done and apprenticing).

I've already said that I am not a programmer, I do not have the mindset. I do what I must and I learn by example.

RTFM replies are useless and elitist. If I understood manuals, I would have said: "can someone point me to the instructions as to how to..." . I didn't, I said "can someone give me the small bit of code that I couldn't figure out on my own".

Did you see the small bit of code that I gave you?
--- rod.

theNbomr: I saw the 'remove whitespace' bit of code you gave me... Thanks.