Check if String Starts with or Ends with Number in JS

# Table of Contents

1. Check if a String starts with a Number using JavaScript
2. Check if a String ends with a Number in JavaScript

# Check if a String starts with a Number using JavaScript

To check if a string starts with a number, call the test() method on a regular expression that matches a number at the beginning of the string.

The test method will return true if the string starts with a number and false otherwise.

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// ✅ Check if a string starts with a number
function startsWithNumber(str) {
  return /^\d/.test(str);
}

console.log(startsWithNumber('avocado 123')); // 👉️ false
console.log(startsWithNumber('456 avocado')); // 👉️ true
console.log(startsWithNumber('0.3 abc')); // 👉️ true

// -------------------------------------------------------

// ✅️ Get the number from the start of the string
function getNumberAtStart(str) {
  if (startsWithNumber(str)) {
    return Number(str.match(/^\d+/)[0]);
  }

  return null;
}

console.log(getNumberAtStart('avocado 123')); // 👉️ null
console.log(getNumberAtStart('456 avocado')); // 👉️ 456
console.log(getNumberAtStart('0.3 abc')); // 👉️ 0

The RegExp.test() method returns true if the regular expression is matched in the string and false otherwise.

The forward slashes / / mark the beginning and end of the regular expression.

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function startsWithNumber(str) {
  return /^\d/.test(str);
}

The caret ^ matches the beginning of the input.

The \d special character matches all digits from 0 to 9.

You can also use the [0-9] character class to achieve the same result.

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function startsWithNumber(str) {
  return /^[0-9]/.test(str);
}

console.log(startsWithNumber('avocado 123')); // 👉️ false
console.log(startsWithNumber('456 avocado')); // 👉️ true
console.log(startsWithNumber('0.3 abc')); // 👉️ true

The [0-9] character class matches the digits in the range.

In its entirety, the regular expression matches one or more digits at the beginning of the string.

# Get the number at the start of a String

If you need to get the number at the start of the string, use the String.match() method.

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function startsWithNumber(str) {
  return /^\d/.test(str);
}

function getNumberAtStart(str) {
  if (startsWithNumber(str)) {
    return Number(str.match(/^\d+/)[0]);
  }

  return null;
}

console.log(getNumberAtStart('avocado 123')); // 👉️ null
console.log(getNumberAtStart('456 avocado')); // 👉️ 456
console.log(getNumberAtStart('0.3 abc')); // 👉️ 0

The String.match() method matches a string against a regular expression.

The method returns an array containing the matches (if any) or null if no matches are found.

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function getNumberAtStart(str) {
  if (startsWithNumber(str)) {
    return Number(str.match(/^\d+/)[0]);
  }

  return null;
}

If the string doesn't start with any digits, the function returns null.

We used our startsWithNumber function to verify that there will be a match before calling the match method.

The last step is to convert the matched string to a number and return the result.

If you ever need help reading a regular expression, check out this regular expression cheat sheet by MDN.

It contains a table with the name and the meaning of each special character with examples.

# Check if a String ends with a Number in JavaScript

To check if a string ends with a number, call the test() method on a regular expression that matches one or more numbers at the end of a string.

The test() method will return true if the string ends with a number and false otherwise.

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// ✅ Check if a String ends with a Number
function endsWithNumber(str) {
  return /[0-9]+$/.test(str);
}

console.log(endsWithNumber('hello 123')); // 👉️ true
console.log(endsWithNumber('123 apple')); // 👉️ false
console.log(endsWithNumber('test 0.5')); // 👉️ true

// -----------------------------------------------------

// ✅ Get the number at end of the string
function getNumberAtEnd(str) {
  if (endsWithNumber(str)) {
    return Number(str.match(/[0-9]+$/)[0]);
  }

  return null;
}

console.log(getNumberAtEnd('hello 123')); // 👉️ 123
console.log(getNumberAtEnd('123 apple')); // 👉️ null
console.log(getNumberAtEnd('test 0.5')); // 👉️ 5

The RegExp.test() method returns true if the regular expression is matched in the string and false otherwise.

The forward slashes / / mark the beginning and end of the regular expression.

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function endsWithNumber(str) {
  return /[0-9]+$/.test(str);
}

The square brackets [] are called a character class and match any digit from 0 to 9.

The plus + matches the preceding item (the range of digits) one or more times.

The dollar $ sign matches the end of the input.

In its entirety, the regular expression matches one or more digits at the end of a string.

You can also replace the [0-9] range with the \d special character to achieve the same result.

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function endsWithNumber(str) {
  return /\d+$/.test(str);
}

console.log(endsWithNumber('hello 123')); // 👉️ true
console.log(endsWithNumber('123 apple')); // 👉️ false
console.log(endsWithNumber('test 0.5')); // 👉️ true

The \d character matches any digit from 0 to 9.

# Get the number from the end of a string

If you need to get the number from the end of the string, use the String.match() method with the same regular expression.

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function endsWithNumber(str) {
  return /[0-9]+$/.test(str);
}

function getNumberAtEnd(str) {
  if (endsWithNumber(str)) {
    return Number(str.match(/[0-9]+$/)[0]);
  }

  return null;
}

console.log(getNumberAtEnd('hello 123')); // 👉️ 123
console.log(getNumberAtEnd('123 apple')); // 👉️ null
console.log(getNumberAtEnd('test 0.5')); // 👉️ 5

The String.match() method matches a string against a regular expression.

The method returns an array containing the matches (if any) or null if no matches are found.

We used the endsWithNumber function to verify that there will be a match in advance before calling the match() method.

Lastly, we convert the extracted value to a number using the Number() constructor.

If you ever need help reading a regular expression, check out this regular expression cheat sheet by MDN.

It contains a table with the name and the meaning of each special character with examples.