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validate username with regex in javascript

I am a newbie to regex and would like to create a regular expression to check usernames. These are the conditions:
username must have between 4 and 20 characters
username must not contain anything but letters a-z, digits 0-9 and special characters -._
the special characters -._ must not be used successively in order to avoid confusion
the username must not contain whitespaces
Examples
any.user.13 => valid
any..user13 => invalid (two dots successively)
anyuser => valid
any => invalid (too short)
anyuserthathasasupersuperlonglongname => invalid (too many characters)
any username => invalid because of the whitespace
I've tried to create my own regex and only got to the point where I specify the allowed characters:
[a-z0-9.-_]{4,20}
Unfortunately, it still matches a string if there's a whitespace in between and it's possible to have two special chars .-_ successively:
If anybody would be able to provide me with help on this issue, I would be extremely grateful. Please keep in mind that I'm a newbie on regex and still learning it. Therefore, an explanation of your regex would be great.
Thanks in advance :)

Sometimes writing a regular expression can be almost as challenging as finding a user name. But here you were quite close to make it work. I can point out three reasons why your attempt fails.
First of all, we need to match all of the input string, not just a part of it, because we don't want to ignore things like white spaces and other characters that appear in the input. For that, one will typically use the anchors ^ (match start) and $ (match end) respectively.
Another point is that we need to prevent two special characters to appear next to each other. This is best done with a negative lookahead.
Finally, I can see that the tool you are using to test your regex is adding the flags gmi, which is not what we want. Particularly, the i flag says that the regex should be case insensitive, so it should match capital letters like small ones. Remove that flag.
The final regex looks like this:
/^([a-z0-9]|[-._](?![-._])){4,20}$/
There is nothing really cryptic here, except maybe for the group [-._](?![-._]) which means any of -._ not followed by any of -._.

Regex allowing only alphanumeric and special characters does not work

I have constructed the following Regex, which allows strings that only satisfy all three conditions:
Allows alphanumeric characters.
Allows special characters defined in the Regex.
String length must be min 8 and max 20 characters.
The Regex is:
"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[$#$!%*?&])[A-Za-z\d$#$!%*?&]$"
I use the following Javascript code to verify input:
var regPassword = new RegExp("^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[$#$!%*?&])[A-Za-z\d$#$!%*?&]$");
regPassword.test(form.passwordField.value);
The test() method returns false for such inputs as abc123!ZXCBN. I have tried to locate the problem in the Regex without any success. What causes the Regex validation to fail?

I see two major problems. One is that inside a string "...", backslashes \ have a special meaning, independent of their special meaning inside a regex. In particular, \d ends up just becoming d — not what you want. The best fix for that is to use the /.../ notation instead of new RegExp("..."):
var regPassword = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[$#$!%*?&])[A-Za-z\d$#$!%*?&]$/;
The other problem is that your regex doesn't match your requirements.
Actually, the requirements that you've stated don't really make sense, but I'm guessing you want something like this:
Must contain at least one lowercase letter, at least one uppercase letter, at least one digit, and at least one of the special characters $#$!%*?&.
Can only contain lowercase letters, uppercase letters, digits, and the special characters $#$!%*?&.
Total length must be between 8 and 20 characters, inclusive.
If so, then you've managed #1 and #2, but forgot about #3. Right now your regex demands that the length be exactly 1. To fix this, you need to add {8,20} after the [A-Za-z\d$#$!%*?&] part:
var regPassword = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[$#$!%*?&])[A-Za-z\d$#$!%*?&]{8,20}$/;

javascript regular expression not working

I am trying to validate a textbox to ensure that a URL is written inside (minus the "http://" part). Here is my code:
var isUrl;
var regex = new RegExp("^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)");
var siteUrl = e.target.value;
if (siteUrl.match(regex)) {
isUrl = true;
}
else {
isUrl = false;
}
So my regular expression is ^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?).
I was under the impression that this would do the following:
NOT match anything beginning with http.. which it does correctly
Allow an optional www. at the start
Accept 1 or more characters from a-z to be typed in
Accept a dot after those characters
Accept 2 or more characters following the dot, and
Allow an optional dot followed by two or more characters again.
In practice, the textbox accepts strings like "ashdiguasdf" and "aksjdnfa:://',~" which it should not do. Can anyone explain why?

The main problem is that you're using the \ character in a quoted string, which javascript will interpret as the start of a "control character" (such as \n for a newline, etc).
One option is to escape it by replacing \ with \\.
But the easiest solution is to use the following format...
var regex = new RegExp(/^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)/);
This also allows you to make it case insensitive (if you wish) by using the i character at the end, like this...
var regex = new RegExp(/^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)/i);
As an extra bit, you're using more capture groups than really necessary. Your expression could also be written like this with the same result...
^(?!http)(?:www\.)?[a-z]+(?:\.[a-z]{2,}){1,2}

What does this JavaScript Regular Expression /[^\d.-] mean?

We had a developer here who had added following line of code to a web application:
var amount = newValue.replace(/[^\d.-]/g, '');
The particular line deals with amount values that a user may enter into a field.
I know the following about the regular expression:
that it replaces the matches with empty strings (i.e. removes them)
that /g is a flag that means to match all occurrences inside "newValue"
that the brackets [] denote a special group
that ^ means beginning of the line
that d means digits
Unfortunately I do not know enough to determine what kind of strings this should match. I checked with some web-based regex testers if it matches e.g. strings like 98.- and other alternatives with numbers but so far no luck.
My problem is that it seems to make IE very slow so I need to replace it with something else.
Any help on this would be appreciated.
Edit:
Thanks to all who replied. I tried not just Google but sites like myregextester.com, regular-expressions.info, phpliveregex.com, and others. My problem was misunderstanding the meaning of ^ and expecting that this required a numeric string like 44.99.

Inside the group, when the ^ is the first character, it works as a negation of the character matches. In other words, it's saying match any character that are not the ones in the group.
So this will mean "match anything that is not a digit, a period, or a hyphen".

The ^ character is a negation character.
var newValue = " x44x.-x ";
var amount = newValue.replace(/[^\d.-]/g, '');
console.log(amount);
will print
44.-
I suspect the developer maybe just wanted to remove trailing whitespaces? I would rather try to parse the string for numbers and remove anything else.

Regex for alphanumeric and special characters in jquery

i tried to make a regex for my Address column the code is:
var str = "97sadf []#-.'";
var regx = /^[a-zA-z0-9\x|]|[|-|'|.]*$/;
if(str.match(regx))
document.write('Correct!');
else
document.write('Incorrect!');
the special character i want that is ][#-. the given code return me the correct match but if i add another kind of the special character like #% then the correct result i got, but i want the incorrect result.
i don't know where i did wrong please help me to make right..
EDIT: Sorry guys but the one thing i have to discuss with you that is there is no necessary to enter the special characters that i mentioned ][#-., but if the someone enter other then the given special character then should return the incorrect.

The correct regex (assuming you want uppercase letters, lowercase letters, numbers, spaces and special characters [].-#') is:
var regx = /^[a-zA-Z0-9\s\[\]\.\-#']*$/
There are a couple things breaking your code.
First, [, ], - and . have special meaning, and must be escaped (prefixed with \).
\x checks for line breaks, where we want spaces (\s).
Next, lets look at the structure; for simplicity's sake, lets simplify to ^[abc]|[def]*$. (abc and def being your two blocks of character types). Since the * is attached to the second block, it is saying one instance of [abc] or any number of [def].
Finally, we don't need | inside of brackets, becuase they already mean one character contained within them (already behaves like an or).