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To detect if a string is a number with regex, you can use the following regular expression:

^-?\d+(\.\d+)?$

Explanation:

• ^ matches the start of the string
• -? matches an optional minus sign (-)
• \d+ matches one or more digits
• (\.\d+)? matches an optional decimal point followed by one or more digits
• $ matches the end of the string

This regular expression will match any string that represents a number, including integers and decimal numbers, with or without a leading minus sign.

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Here’s an example of how to use this regular expression to check if a string is a number in JavaScript:

function isNumber(str) {
  return /^-?\d+\.?\d*$/.test(str);
}

console.log(isNumber('42')); // true
console.log(isNumber('-3.14')); // true
console.log(isNumber('0')); // true
console.log(isNumber('4.2.1')); // false
console.log(isNumber('hello')); // false

The test method returns true if the regular expression matches the given string, and false otherwise.