JavaScript Regex Pattern Concatenate with Variable

JavaScript regex pattern concatenate with variable

var re = new RegExp("/\b"+test+"\b/"); 

\b in a string literal is a backspace character. When putting a regex in a string literal you need one more round of escaping:

var re = new RegExp("\\b"+test+"\\b"); 

(You also don't need the // in this context.)

Use dynamic (variable) string as regex pattern in JavaScript

To create the regex from a string, you have to use JavaScript's RegExp object.

If you also want to match/replace more than one time, then you must add the g (global match) flag. Here's an example:

var stringToGoIntoTheRegex = "abc";
var regex = new RegExp("#" + stringToGoIntoTheRegex + "#", "g");
// at this point, the line above is the same as: var regex = /#abc#/g;

var input = "Hello this is #abc# some #abc# stuff.";
var output = input.replace(regex, "!!");
alert(output); // Hello this is !! some !! stuff.

JSFiddle demo here.

In the general case, escape the string before using as regex:

Not every string is a valid regex, though: there are some speciall characters, like ( or [. To work around this issue, simply escape the string before turning it into a regex. A utility function for that goes in the sample below:

function escapeRegExp(stringToGoIntoTheRegex) {
    return stringToGoIntoTheRegex.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
}

var stringToGoIntoTheRegex = escapeRegExp("abc"); // this is the only change from above
var regex = new RegExp("#" + stringToGoIntoTheRegex + "#", "g");
// at this point, the line above is the same as: var regex = /#abc#/g;

var input = "Hello this is #abc# some #abc# stuff.";
var output = input.replace(regex, "!!");
alert(output); // Hello this is !! some !! stuff.

JSFiddle demo here.

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Note: the regex in the question uses the s modifier, which didn't exist at the time of the question, but does exist -- a s (dotall) flag/modifier in JavaScript -- today.

concat variable in regexp pattern

You need to build your regex as a string and pass it to the RegExp constructor:

var regexString = '\\d{' + number + '}';
var regex = new RegExp(regexString);
var p = str.match(regex);

Notice that when building a regex via a string, you need to add some extra escape characters to escape the string as well as the regex.

How to concat a regex expression with a string variable?

You don't need to add / at the beginning and the end of new RegExp(...) and \ should be escaped as mentioned by anubhava :

var decimal = new RegExp("^\\s*(\\+|-)?((\\d+(" + myVar + "\\d+)?)|(" + myVar + "\\d+))\\s*$");

How to concat two javascript variables and regex expression

Try this (I use nodejs):

> var t1 = "Test1"
> var t2 = "Test2"
> var re = new RegExp('^' + t1 + '.*' + t2 + '$')
> re
/^Test1.*Test2$/
> re.test("Test1  this works   Test2")
true

Note

• .* as stated in comments, this means any character repeated from 0 to ~
• the slashes are automagically added when calling the RegExp constructor, but you can't have nested unprotected slashes delimiters
• to ensure Test1 is at the beginning, i put ^ anchor, and for Test2 at the end, I added $ anchor
• the regex constructor is not ReGex but RegExp (note the trailing p)

Is it Possible to Concatenate variable in Split Regexp in Javascript? Eg: var.split(/[A-Z]/+ variable + / \d/);

You can compile a regular expression from a variable, e.g.:

var regex = new RegExp();

var src = "\\s"; // don't forget to escape slashes!
var mods  = "g";
regex.compile(src, mods); 

alert(regex.source);

How can I concatenate regex literals in JavaScript?

Here is how to create a regular expression without using the regular expression literal syntax. This lets you do arbitary string manipulation before it becomes a regular expression object:

var segment_part = "some bit of the regexp";
var pattern = new RegExp("some regex segment" + /*comment here */
              segment_part + /* that was defined just now */
              "another segment");

If you have two regular expression literals, you can in fact concatenate them using this technique:

var regex1 = /foo/g;
var regex2 = /bar/y;
var flags = (regex1.flags + regex2.flags).split("").sort().join("").replace(/(.)(?=.*\1)/g, "");
var regex3 = new RegExp(expression_one.source + expression_two.source, flags);
// regex3 is now /foobar/gy

It's just more wordy than just having expression one and two being literal strings instead of literal regular expressions.

Properly escape variable inside RegEx (JavaScript)

You do not need / and you can do without the capturing group:

var regex_string = "^\\w{" + highlight_chars_count + "}";

and then

return v.replace(regex, "<span>$&</span>"); 
                                ^

Note that the regex delimiters (/.../) are necessary when you declare a regex with a regex literal notation when a regex is static (e.g. var rx = /abc/g). Here, you use a constructor notation.

Also, $& backreference refers to the whole match text, so, no need enclosing the whole pattern with a capturing group.

More information on RegExp regex literal and constructor notation at MDN

How can i concatenate in javascript?

It looks like you're trying to use a variable in a regular expression literal. I don't think the interpreter would understand that. However, you can pass a string to the regular expression constructor, and strings can be concatenated/interpolated all you like.

For example:

var name = 'test';
var query = new RegExp('^ ' + name);

or:

var name = 'test';
var query = new RegExp(`^ ${name}`);

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