JavaScript - How to Use a Variable in Regular Expression?

To dynamically create a regular expression (RegExp) in JavaScript using variables, you can use the RegExp constructor. Here are the various ways to use a variable in Regular Expression.

1. Using the RegExp Constructor with a Variable

In JavaScript, regular expressions can be created dynamically using the RegExp constructor. This method allows you to pass a string as the pattern and optionally specify flags for the regular expression.

let search = "hello";
let regex = new RegExp(search, "i"); // Case-insensitive search
let s = "Hello, how are you?";

let res = regex.test(s);
console.log(res); 

true

• search: A variable that contains the word you want to search for in the string.
• new RegExp(search, "i"): This dynamically creates a regular expression based on the search variable. The "i" flag makes the search case-insensitive.
• test(str): The test() method checks if the regular expression matches the string str.

2. Using Variables to Create More Complex Patterns

You can use variables to dynamically create more complex regular expressions as well, such as matching multiple words or patterns.

let w1 = "apple";
let w2 = "banana";
let regex = new RegExp(w1 + "|" + w2, "i");

let s = "I like both apple and banana.";
let res = regex.test(s);
console.log(res);

true

• w1 + "|" + w2: This creates a pattern that matches either "apple" or "banana". The | symbol is used as an OR operator in regular expressions.
• new RegExp(w1 + "|" + w2, "i"): Dynamically creates the regular expression.
• test(): Checks if the pattern matches the string.

3. Using Variables for Flags in Regular Expressions

You can also dynamically set the flags of a regular expression. This is useful if you want to apply different search options based on certain conditions.

let pattern = "hello";
let flag = "i";
let regex = new RegExp(pattern, flag);

let s = "Hello world!";
let res = regex.test(s);
console.log(res);

true

• flag = "i": The flag is stored in a variable, which you can change dynamically.
• new RegExp(pattern, flag): The regular expression is constructed using both the pattern and the flag variable.
• test(): The method checks if the regular expression matches the string.

4. Special Case: Escaping Special Characters in Variables

When using a variable in a regular expression, you might need to escape special characters (like ., *, +, etc.) that could interfere with the regular expression syntax.

let pattern = "hello.world";
let escapedPat = pattern.replace(/[.*+?^=!:${}()|\[\]\/\\]/g, "\\$&");
let regex = new RegExp(escapedPat);

let s = "hello.world";
let res = regex.test(s);
console.log(res);

true

• replace(/[.*+?^=!:${}()|\[\]\/\\]/g, "\\$&"): This line escapes any special characters in the pattern. Special characters in regular expressions need to be escaped to match them literally.
• new RegExp(escapedPat): After escaping special characters, the escapedPat is used to create a valid regular expression.

Use Cases for Using Variables in Regular Expressions

• Dynamic Searching: When you need to match a pattern that changes based on user input or some other dynamic data (like matching a search term).
• Form Validation: Dynamically generate regular expressions based on form input values to validate things like email, phone numbers, or passwords.
• Text Processing: When processing text files or strings dynamically, you can use variables to build the search pattern based on the data.