Develop Reference

How not to match a certain regexp in javascript?

I want to check if a variable do not match this regexp:
DEMO
So this is the pattern that match the regexp in my code:
rxAfterPrint = new RegExp(/^ *\+ *("(?:[^"]*)"|(?:[a-zA-Z]\w*)) *(.*)$/);
and in this way I check for matching:
var t2 = t[2].match(rxAfterPrint);
and now I want to create e varible t3 that dont match this pattern
How can I do this? can you please help me?

(Admitting I have an unfair advantage because I knew why this problem did arise: How can I interpret strings in textarea with JavaScript/jQuery?)
So my guess is you want to implement String concatenation as part of a print statement as follows:
<string> ::= '"' <character>* '"' | <variable>
<print> ::= 'print' <string> ('+' <string>)*
<print> ::= 'print' (<string> '+')* <string>
The two <print> actually express the same, using the 2nd version you can first (after matching /^ *print */) try to apply the pattern rxConcat as many times a possible and if this doesn't match, then you apply the 2nd expression rxStringValEOL to match the remainder (if no success, it's an invalid statement):
rxConcat = new RegExp(/ *(?:"([^"]*)"|([a-zA-Z]\w*)) *\+ */);
rxStringValEOL = new RegExp(/ *(?:"([^"]*)"|([a-zA-Z]\w*)) *$/);
This also shows that it is pretty difficult to design a language that is easy for the programmers and for those who write the compilers.

It's really unclear what you mean by "I want to create a variable that don't match this pattern". Since t2 is your match, it seems like you want t3 to be objects that don't match.
Because you're anchoring to the start of the string (^), this is a really great place to use a negative lookahead with almost the identical regex. Literally, all I did was surround it with (?! and ) and .* at the end..
output1.value = input.value.match(/^(?! *\+ *("(?:[^"]*)"|(?:[a-zA-Z]\w*)) *(.*)).*$/gm).join("\r\n")
An alternative is to use replace() like so, but I would believe match() is the better option.
output2.value = input.value.replace(/(^ *\+ *("(?:[^"]*)"|(?:[a-zA-Z]\w*)) *(.*)$\s*)+/gm,"")
For both cases, I added the global and multiline to easily test several lines at once. If you're only testing one, remove both the g and the m, otherwise it could cause bugs by incorrectly telling you a string passed or failed when it didn't.
Demo: JSFiddle

Simple Regexp Pattern matching with escape characters

Hopefully a simple one!
I've been trying to get this to work for several hours now but am having no luck, as I'm fairly new to regexp I may be missing something very obvious here and was hoping someone could point me in the right direction. The pattern I want to match is as follows: -
At least 1 or more numbers + "##" + at least 1 or more numbers + "##" + at least 1 or more numbers
so a few examples of valid combinations would be: -
1##2##3
123#123#123
0##0##0
A few invalid combinations would be
a##b##c
1## ##1
I've got the following regexp like so: -
[\d+]/#/#[\d+]/#/#[\d+]
And am using it like so (note the double slashes as its inside a string): -
var patt = new RegExp("[\\d+]/#/#[\\d+]/#/#[\\d+]");
if(newFieldValue!=patt){newFieldValue=="no match"}
I also tried these but still nothing: -
if(!patt.text(newFieldValue)){newFieldValue==""}
if(patt.text(newFieldValue)){}else{newFieldValue==""}
But nothing I try is matching, where am I going wrong here?
Any pointers gratefully received, cheers!

1) I can't see any reason to use the RegExp constructor over a RegExp literal for your case. (The former is used primarily where the pattern needs to by dynamic, i.e. is contributed to by variables.)
2) You don't need a character class if there's only one type of character in it (so \d+ not [\d+]
3) You are not actually checking the pattern against the input. You don't apply RegEx by creating an instance of it and using ==; you need to use test() or match() to see if a match is made (the former if you want to check only, not capture)
4) You have == where you mean to assign (=)
if (!/\d+##\d+##\d+/.test(newFieldValue)) newFieldValue = "no match";

You put + inside the brackets, so you're matching a single character that's either a digit or +, not a sequence of digits. I also don't understand why you have / before each #, your description doesn't mention anything about this character.
Use:
var patt = /\d+##\d+##\d+/;

You should use the test method of the pat regex
if (!patt.test(newFieldValue)){ newFieldValue=="no match"; }
once you have a valid regular expression.

Try this regex :
^(?:\d+##){2}\d+$
Demo: http://regex101.com/r/mE8aG7
With the following regex
[\d+]/#/#[\d+]/#/#[\d+]
You would only match things like:
+/#/#5/#/#+
+/#/#+/#/#+
0/#/#0/#/#0
because the regex engine sees it like on the schema below:

Something like:
((-\s)?\d+##)+\d+

Regular Expression - Match String Not Preceded by Another String (JavaScript)

I am trying to find a regular expression that will match a string when it's NOT preceded by another specific string (in my case, when it is NOT preceded by "http://"). This is in JavaScript, and I'm running on Chrome (not that it should matter).
The sample code is:
var str = 'http://www.stackoverflow.com www.stackoverflow.com';
alert(str.replace(new RegExp('SOMETHING','g'),'rocks'));
And I want to replace SOMETHING with a regular expression that means "match www.stackoverflow.com unless it's preceded by http://". The alert should then say "http://www.stackoverflow.com rocks", naturally.
Can anyone help? It feels like I tried everything found in previous answers, but nothing works. Thanks!

As JavaScript regex engines don't support 'lookbehind' assertions, it's not possible to do with plain regex. Still, there's a workaround, involving replace callback function:
var str = "As http://JavaScript regex engines don't support `lookbehind`, it's not possible to do with plain regex. Still, there's a workaround";
var adjusted = str.replace(/\S+/g, function(match) {
return match.slice(0, 7) === 'http://'
? match
: 'rocks'
});
console.log(adjusted);
You can actually create a generator for these functions:
var replaceIfNotPrecededBy = function(notPrecededBy, replacement) {
return function(match) {
return match.slice(0, notPrecededBy.length) === notPrecededBy
? match
: replacement;
}
};
... then use it in that replace instead:
var adjusted = str.replace(/\S+/g, replaceIfNotPrecededBy('http://', 'rocks'));
JS Fiddle.

raina77ow's answer reflected the situation in 2013, but it is now outdated, as the proposal for lookbehind assertions got accepted into the ECMAScript spec in 2018.
See docs for it on MDN:
Characters
Meaning
(?<!y)x
Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y". For example, /(?<!-)\d+/ matches a number only if it is not preceded by a minus sign. /(?<!-)\d+/.exec('3') matches "3". /(?<!-)\d+/.exec('-3') match is not found because the number is preceded by the minus sign.
Therefore, you can now express "match www.stackoverflow.com unless it's preceded by http://" as /(?<!http:\/\/)www.stackoverflow.com/:
const str = 'http://www.stackoverflow.com www.stackoverflow.com';
console.log(str.replace(/(?<!http:\/\/)www.stackoverflow.com/g, 'rocks'));

This also works:
var variable = 'http://www.example.com www.example.com';
alert(variable.replace(new RegExp('([^(http:\/\/)|(https:\/\/)])(www.example.com)','g'),'$1rocks'));
The alert says "http://www.example.com rocks".

Add regex to ignore /js /img and /css

I have this regular expression
// Look for /en/ or /en-US/ or /en_US/ on the URL
var matches = req.url.match( /^\/([a-zA-Z]{2,3}([-_][a-zA-Z]{2})?)(\/|$)/ );
Now with the above regular express it will cause the problem with the URL such as:
http://mydomain.com/css/bootstrap.css
or
http://mydomain.com/js/jquery.js
because my regular expression is to strip off 2-3 characters from A-Z or a-z
My question is how would I add in to this regular expression to not strip off anything with
js or img or css or ext
Without impacting the original one.
I'm not so expert on regular expression :(

Negative lookahead?
var matches = req.url.match(/^\/(?!(js|css))([a-zA-Z]{2,3}([-_][a-zA-Z]{2})?)(\/|$)/ );
\ not followed by js or css

First of all you have not defined what exactly you are searching for.
Define an array with lowercased common language codes (Common language codes)
This way you'll know what to look for.
After that, convert your url to lowercase and replace all '_' with '-' and search for every member of the array in the resulting string using indexOf().

Since you said you're using the regex to replace text, I changed it to a replace function. Also, you forced the regex to match the start of the string; I don't see how it would match anything with that. Anyway, here's my approach:
var result = req.url.replace(/\/([a-z]{2,3}([-_][a-z]{2})?)(?=\/|$)/i,
function(s,t){
switch(t){case"js":case"img":case"css":case"ext":return s;}
return "";
}
);

Is there a way to do a substring in Javascript but use string characters as the parameters for what you want to select?

So a substring can take two parameters, the index to start at and the index to stop at like so
var str="Hello beautiful world!";
document.write(str.substring(3,7));
but is there a way to designate the start and stopping points as a set of characters to grab, so instead of the starting point being 3 I would want it to be "lo" and instead of the end point being 7 I would want it to be "wo" so I would be grabbing "lo beautiful wo". Is there a Javascript function that serves that purpose already?

Sounds like you want to use regular expressions and string.match() instead:
var str="Hello beautiful world!";
document.write(str.match(/lo.*wo/)[0]); // document.write("lo beautiful wo");
Note, match() returns an array of matches, which might be null if there is no match. So you should include a null check.
If you're not familiar with regexes, this is a pretty good source:
http://www.w3schools.com/jsref/jsref_obj_regexp.asp

use the method indexOf: document.write(str.substring(3,str.indexOf('wo')+2));

Yup, you can do this easily with regular expressions:
var substr = /lo.+wo/.exec( 'Hello beautiful world!' )[0];
console.log( substr ); //=> 'lo beautiful wo'

Use a regex brother:
if (/(lo.+wo)/.test("Hello beautiful world!")) {
document.write(RegExp.$1);
}
You need a backup plan in case the string does not match. Hence the use of test.

Regular expression may be able to achieve this to some extent, but there are many details that you must be aware of.
For example, if you want to find all the substrings that starts with "lo", and ends with the nearest "wo" after "lo". (If there are more than 1 match, the subsequent matches will pick up the first "lo" after the "wo" of last match).
"Hello beautiful world!".match(/lo.*?wo/g);
Using the RegExp constructor, you can make it more flexible (you can substitute "lo" and "wo" with the actual string you want to find):
"Hello beautiful world!".match(new RegExp("lo" + ".*?" + "wo", "g"));
Important: The downside of the RegExp approach above is that, you need to know what characters are special to escape them - otherwise, they will not match the actual substring you want to find.
It can also be achieve with indexOf, albeit a little bit dirty. For the first substring:
var startIndex = str.indexOf(startString);
var endIndex = str.indexOf(endString, startIndex);
if (startIndex >= 0 && endIndex >= 0)
str.substring(startIndex, endIndex + endString.length)
If you want to find the substring that starts with the first "lo" and ends with the last "wo" in the string, you can use indexOf and lastIndexOf to find it (with a small modification to the code above). RegExp can also do it, by changing .*? to .* in the two example above (there will be at most 1 match, so the "g" flag at the end is redundant).