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regex custom lenght but no whitespace allowed [duplicate]

I have a username field in my form. I want to not allow spaces anywhere in the string. I have used this regex:
var regexp = /^\S/;
This works for me if there are spaces between the characters. That is if username is ABC DEF. It doesn't work if a space is in the beginning, e.g. <space><space>ABC. What should the regex be?

While you have specified the start anchor and the first letter, you have not done anything for the rest of the string. You seem to want repetition of that character class until the end of the string:
var regexp = /^\S*$/; // a string consisting only of non-whitespaces

Use + plus sign (Match one or more of the previous items),
var regexp = /^\S+$/

If you're using some plugin which takes string and use construct Regex to create Regex Object i:e new RegExp()
Than Below string will work
'^\\S*$'
It's same regex #Bergi mentioned just the string version for new RegExp constructor

This will help to find the spaces in the beginning, middle and ending:
var regexp = /\s/g

This one will only match the input field or string if there are no spaces. If there are any spaces, it will not match at all.
/^([A-z0-9!##$%^&*().,<>{}[\]<>?_=+\-|;:\'\"\/])*[^\s]\1*$/
Matches from the beginning of the line to the end. Accepts alphanumeric characters, numbers, and most special characters.
If you want just alphanumeric characters then change what is in the [] like so:
/^([A-z])*[^\s]\1*$/

Where am I going wrong using regex?

var s = "07:05:45PM";
var meridian = s.match("/[AM PM]/i"); //matches AM or PM in s
console.log(meridian);
// Expecting output as [PM] but actual output is null
Trying to get whether it is AM or PM. I am not sure where I am going wrong.

Your problem is really made up of two problems: your regular expression syntax and the value you're passing to match.
Placing text inside of square brackets in a regular expression (e.g. [AM PM]) means "match any one character that is declared in here." So doing [AM PM] translates to "Match an A, a P, an M, or a space." You would usually see such an expression written as [AMP ] (or any order of those characters). To match what you're looking for, try something like this:
[AP]M
which means "match either an A or a P followed by an M".
Then there's the problem of what you're passing to match. match should take a regular expression literal, not a string. Basically, remove the quotation marks.
Your final code could look like this:
var s = "07:05:45PM";
var meridian = s.match(/[AP]M/i); //matches AM or PM in s
console.log(meridian);

You need a regular expression, not a string.
var s = "07:05:45PM";
var meridian = s.match(/[AP]M/i); //matches AM or PM in s
console.log(meridian);

You should be using groups not character sets.
[AM PM] matches a single character in that set (A or M or P or ).
(AM|PM) matches the entire string where | is an or. In the example, it will match AM or PM.
Second, the input to match should be a regular expression not a string (remove the double quotes).

Try one of these
/[AP]M/i or /(AM|PM)/i
Your current regex is matching anything with only one the letters [AMP ].
Also, don't wrap your regex in quotes, you want it to be a regex not a string.

Brackets in Regular Expression

I'd like to compare 2 strings with each other, but I got a little problem with the Brackets.
The String I want to seek looks like this:
CAPPL:LOCAL.L_hk[1].vorlauftemp_soll
Quoting those to bracket is seemingly useless.
I tried it with this code
var regex = new RegExp("CAPPL:LOCAL.L_hk\[1\].vorlauftemp_soll","gi");
var value = "CAPPL:LOCAL.L_hk[1].vorlauftemp_soll";
regex.test(value);
Somebody who can help me??

It is useless because you're using string. You need to escape the backslashes as well:
var regex = new RegExp("CAPPL:LOCAL.L_hk\\[1\\].vorlauftemp_soll","gi");
Or use a regex literal:
var regex = /CAPPL:LOCAL.L_hk\[1\].vorlauftemp_soll/gi
Unknown escape characters are ignored in JavaScript, so "\[" results in the same string as "[".

In value, you have (1) instead of [1]. So if you expect the regular expression to match and it doesn't, it because of that.
Another problem is that you're using "" in your expression. In order to write regular expression in JavaScript, use /.../g instead of "...".
You may also want to escape the dot in your expression. . means "any character that is not a line break". You, on the other hand, wants the dot to be matched literally: \..

You are generating a regular expression (in which [ is a special character that can be escaped with \) using a string (in which \ is a special character).
var regex = /CAPPL:LOCAL.L_hk\[1\].vorlauftemp_soll/gi;

Regex from character until end of string

Hey. First question here, probably extremely lame, but I totally suck in regular expressions :(
I want to extract the text from a series of strings that always have only alphabetic characters before and after a hyphen:
string = "some-text"
I need to generate separate strings that include the text before AND after the hyphen. So for the example above I would need string1 = "some" and string2 = "text"
I found this and it works for the text before the hyphen, now I only need the regex for the one after the hyphen.
Thanks.

You don't need regex for that, you can just split it instead.
var myString = "some-text";
var splitWords = myString.split("-");
splitWords[0] would then be "some", and splitWords[1] will be "text".
If you actually have to use regex for whatever reason though - the $ character marks the end of a string in regex, so -(.*)$ is a regex that will match everything after the first hyphen it finds till the end of the string. That could actually be simplified that to just -(.*) too, as the .* will match till the end of the string anyway.

Replacing second part of string with Javascript and replace()

I need to replace any occurrence of a sequence of integers followed by a dash and then another sequence of integers, with only the first sequence of integers. For example:
THIS IS A STRING 2387263-1111 STRING CONTINUES
Will become:
THIS IS A STRING 2387263 STRING CONTINUES
Can I use that with Javascript and replace()?

You can do:
str = str.replace(/(\d+)-\d+/,'$1');
See it
Which replaces a group of digits followed by a hyphen followed by a group of digits with the first group of digits.
If you want to replace multiple occurrences of such pattern just use the g modifier as:
str = str.replace(/(\d+)-\d+/g,'$1');

NEW ANSWER -
Yes, in your case according to me, first you need to match that whole string "2387263-1111" using a regex and then remove that part followed by '-' and then replace the result in the original string.
Check the answer from codaddict. Mine would've almost been same but his answer seems more appropriate.
OLD ANSWER: -
Why replace? Just use split and get the first value.
var str = "2387263-1111";
var output = str.split("-")[0];

User RegExp function of javascript
str = str.replace(new RegExp("-[0-9]+"), " ");

Not really a new answer, just an addition to codaddict. Don't know JScript's regex
all that well, but I asume it uses extended regular expressions (if not, forget this).
If you need validation on boundry conditions you could do something like this:
str = str.replace( /((^|\s)\d+)-\d+(?=\s|$)/g, '$1' );
That would prevent matching this type of thing:
A STRING 2387263-1111STRING CONTINUES
A STRING2387263-1111 STRING CONTINUES
A STRING2387263-1111STRING CONTINUES