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Convert user input string into regular expression using JavaScript

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  • Last Updated : 04 Jun, 2020

Regular expressions (RegExp) are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. There are two ways to construct a regular expression in JavaScript .

  • 1. Using a regular expression literal, which consists of a pattern enclosed between slashes, as follows.
const reg = /ab+/;
  • 2. Calling the constructor function of the RegExp object, as follows.
const reg = new RegExp('ab+', flag);

Using the constructor function provides run time compilation of the regular expression, hence we should use the second method here as the string is a dynamic input from the user. Both the above expressions correspond to the same RegExp.  

Example:  For the string we will take “Geeks For Geeks'”.

Input : '^Ge'
Output: ["Ge"]
        0: "Ge" 
Input : '[A-z]+'
Output: (3) ["Geeks", "For", "Geeks"]
        0: "Geeks"
        1: "For"
        2: "Geeks" 


    const str = "Geeks for Geeks";
    // Input from User
    const regex = prompt("Enter RegExp");
    // Conversion from string to RegExp
    const reg = new RegExp(regex, "g");
    // The match fn returns the array of strings 
    // That match to RegExp
    const result = str.match(reg);
    if (result) console.log(result);
    else console.log("Not Found");


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