How to pass a variable into a regExp object in JavaScript and make it work

Question

I have a problem I can not solve. Let's say I have a variable that keeps the regExp pattern, that is to be provided by the user:

var pattern = this.state.regExpVal;

I also have a variable that keeps the value of textInput, meaning some piece of text, eg, postal codes.

var str = this.state.textAreaVal;

I create a new regExp object:

var myRegEx = new RegExp(pattern, 'g');

And the result is not ok:(it seems that the flag search globally is not working and I can not figure out why);

var result = str.match(myRegEx);

Can anyone help?

Both string and regex are to be provided by the user. Let's say that var str='34-990' and pattern for regExp=\d{2}-\d{3}. — LostInJavaScript
– LostInJavaScript, Commented Mar 23, 2017 at 17:22

Dennis Kronbügel · Accepted Answer · 2017-03-23 17:39:35Z

1

let reg = new RegExp("super", 'g');
"SOsuperISsuper!".match(reg);

The result is an Array with 2 Elements ["super","super"] as expected.

I guess the problem lies within your this some undefined or null values or in case-sense (try include 'i')

Edit:

let myObject = {
    state: {
     regExpVal: "super"
    }
}
let reg = new RegExp(myObject.state.regExpVal, 'g');
"SOsuperISsuper!".match(reg);

Edit 2:

let myObject = {
    state: {
     regExpVal: "super"
       }
   };
let pattern = myObject.state.regExpVal;
let reg = new RegExp(pattern, 'g');
"SOsuperISsuper!".match(reg);

edited Mar 23, 2017 at 17:39

answered Mar 23, 2017 at 17:23

Dennis Kronbügel

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4 Comments

LostInJavaScript Over a year ago

Thanks but I want to know whether there is an option to put variables into the pattern. var pattern = this.state.regExpVal; var str = this.state.textAreaVal; var reg = new RegExp(pattern, 'g');-----here trying str.match(reg) is not working ok. Both pattern and str are strings.

LostInJavaScript Over a year ago

I guess maybye the problem lays somewhere else, because I have been trying all combinations. Thank you for the support.

Dennis Kronbügel Over a year ago

You can use variables, but if your value, who feeds the patter variable, is changing, you have to create a new RegEx Object.

Dennis Kronbügel Over a year ago

insert a debugger; statement and inspect your variables at runtime. "this" is tricky for beginners.

dietary-wheevil · Accepted Answer · 2017-03-24 02:26:32Z

I think it would benefit you to provide a few concrete examples. It's difficult to help as well as would be possible without such. However, I can provide examples of how one constructs a regular expression from variables.

Let's assume your variable from which you wish to construct an "on-the-fly" regular expression matching pattern is

const someVar = 'LostInJavaScript';

Use of the Regular Expression constructor, as you seem to be using, is the necessary approach to take here. Inside the constructor, I tend to favor using ES2015-style string templates for the first argument passed it:

const regPattern = new RegExp(`${someVar}`, 'g');
// => /LostInJavaScript/g

Apply the relevant/appropriate String method (e.g., String#match, String#search, RegExp#test, RegExp#exec) passing it this dynamically created regular expression.

const strInput = 'RegularExpressionsHaveMeLostInJavaScript...',
      matchedSubStr = strInput.match(regPattern);
// => ["LostInJavaScript"]

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