Develop Reference

Transform a "Regex" string to actual Regex in Javascript [duplicate]

This question already has answers here:
Converting user input string to regular expression
(14 answers)
Closed 3 years ago.
I need to pass a regular expression in a validation function in my code that can only be set by an administrator in a back office platform as a string (e.g. '/^(?:\d{8}|\d{11})$/'). After it is passed I need to take this string and transform it into an actual javascript regex in order to use it.
const validator = (regex, value) => {
if (value && regex.test(value)) {
return 'Yeah!!';
}
return null;
};
So I need this '/^(?:\d{8}|\d{11})$/' to be like this /^(?:\d{8}|\d{11})$/.

You can initialize a regex with the RegExp method (documentation on MDN):
The RegExp constructor creates a regular expression object for matching text with a pattern.
const regex2 = new RegExp('^(?:\\d{8}|\\d{11})$');
console.log(regex2); // /^(?:\d{8}|\d{11})$/

You could instantiate the RegExp class and use it in two ways:
First:
new RegExp('<expression>').test(...)
Second:
/<expression>/.test(...)
Both ways will create an RegExp instance.
When to use one over the other?
Using the new RegExp way you can pass variables to the expression, like that:
new RegExp('^abc' + myVar + '123$');
Which you can't do using the second way.

How do I insert something at a specific character with Regex in Javascript [duplicate]

This question already has answers here:
Simple javascript find and replace
(6 answers)
Closed 5 years ago.
I have string "foo?bar" and I want to insert "baz" at the ?. This ? may not always be at the 3 index, so I always want to insert something string at this ? char to get "foo?bazbar"

The String.protype.replace method is perfect for this.
Example
let result = "foo?bar".replace(/\?/, '?baz');
alert(result);
I have used a RegEx in this example as requested, although you could do it without RegEx too.
Additional notes.
If you expect the string "foo?bar?boo" to result in "foo?bazbar?boo" the above code works as-is
If you expect the string "foo?bar?boo" to result in "foo?bazbar?bazboo" you can change the call to .replace(/\?/g, '?baz')

You don't need a regular expression, since you're not matching a pattern, just ordinary string replacement.
string = 'foo?bar';
newString = string.replace('?', '?baz');
console.log(newString);

simple regular expression with equals but dont want to use split [duplicate]

This question already has answers here:
Regular Expressions in JavaScript for URL Capture
(4 answers)
Closed 7 years ago.
I have the message string as follows.
string=052
I need to use regular expression not split.
I want to return everything past the equals. 052
This is what i tried and gives me id=null
var regex = '[^?string=]';
var id2 = mystring.match(regex);
I have tried online regex checkers and it looks like it matches all but the a
is there a better reg ex i should try? id should not equal null.

You're using String.match() incorrectly. Try this:
var regex = '^message=(.*)$';
var id = queryString.match(regex)[1];
.match() returns an array; the first element (at [0]) is the entire matched string, and the second element (at [1]) is the part that's matched in the (first) set of parentheses in the regex.

How to extract numbers from string in Javascript using regex [duplicate]

This question already has answers here:
Find and get only number in string
(4 answers)
Closed 8 years ago.
I have the following string
/Date(1317772800000)/
I want to use a Javascript regular expression to extract the numerical portion of it
1317772800000
How is this possible?

That should be it
var numPart = "/Date(1317772800000)/".match(/(\d+)/)[1];

No need for regex. Use .substring() function. In this case try:
var whatever = "/Date(1317772800000)/";
whatever = whatever.substring(6,whatever.length-2);

This'll do it for you: http://regex101.com/r/zR0wH4
var re = /\/Date\((\d{13})\)\//;
re.exec('/Date(1317772800000)/');
=> ["/Date(1317772800000)/", "1317772800000"]
If you don't care about matching the date portion of the string and just want extract the digits from any string, you can use this instead:
var re = /(\d+)/;
re.exec('/Date(1317772800000)/')
["1317772800000", "1317772800000"]

JS replacing all occurrences of string using variable [duplicate]

This question already has answers here:
How do you use a variable in a regular expression?
(27 answers)
Closed 9 years ago.
I know that str.replace(/x/g, "y")replaces all x's in the string but I want to do this
function name(str,replaceWhat,replaceTo){
str.replace(/replaceWhat/g,replaceTo);
}
How can i use a variable in the first argument?

The RegExp constructor takes a string and creates a regular expression out of it.
function name(str,replaceWhat,replaceTo){
var re = new RegExp(replaceWhat, 'g');
return str.replace(re,replaceTo);
}
If replaceWhat might contain characters that are special in regular expressions, you can do:
function name(str,replaceWhat,replaceTo){
replaceWhat = replaceWhat.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
var re = new RegExp(replaceWhat, 'g');
return str.replace(re,replaceTo);
}
See Is there a RegExp.escape function in Javascript?

The third parameter of flags below was removed from browsers a few years ago and this answer is no longer needed -- now replace works global without flags
Replace has an alternate form that takes 3 parameters and accepts a string:
function name(str,replaceWhat,replaceTo){
str.replace(replaceWhat,replaceTo,"g");
}
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace