To check if a string is a valid hashtag, we can use a regex expression to match for a string that starts with a # (hash symbol) in JavaScript.

TL;DR

// Regular expression to check if string is a hashtag
const regexExp = /^#[^ !@#$%^&*(),.?":{}|<>]*$/gi;

// String with hashtag
const str = "#helloWorld";

regexExp.test(str); // true

This is the regex expression for matching almost all the test cases for a valid hashtag in JavaScript.

// Regular expression to check if string is a hashtag
const regexExp = /^#[^ !@#$%^&*(),.?":{}|<>]*$/gi;

Now let's write a string with valid hashtag like this,

// Regular expression to check if string is a hashtag
const regexExp = /^#[^ !@#$%^&*(),.?":{}|<>]*$/gi;

// String with hashtag
const str = "#helloWorld";

Now to test the string, we can use the test() method available in the regular expression we defined. It can be done like this,

// Regular expression to check if string is a hashtag
const regexExp = /^#[^ !@#$%^&*(),.?":{}|<>]*$/gi;

// String with hashtag
const str = "#helloWorld";

regexExp.test(str); // true

• The test() method will accept a string type as an argument to test for a match.
• The method will return boolean true if there is a match using the regular expression and false if not.

See the above example live in JSBin.

If you want this as a utility function which you can reuse, here it is,

/* Check if string is a valid Hashtag */
function checkIfValidHashtag(str) {
  // Regular expression to check if string is a hashtag
  const regexExp = /^#[^ !@#$%^&*(),.?":{}|<>]*$/gi;

  return regexExp.test(str);
}

// Use the function
checkIfValidHashtag("#helloWorld"); // true
checkIfValidHashtag("#%hello98123!"); // false

That's all! 😃

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