Why is -1 judged to be successful when using the regular expression /\[1-9\]\\d\*$/?

Misconceptions and corrections for regular expression matching

This article discusses a common problem in string matching of regular expressions: Why does regular expression /[1-9]\d*$/ determine "-1" as a successful match? And how to fix this problem.

Let's analyze the code:

 function isPositiveInteger(str) {
    const regex = /[1-9]\d*$/;
    return regex.test(str);
}

console.log(isPositiveInteger("-1")); // Output true, expected to be false
console.log(isPositiveInteger("1")); // Output true

The isPositiveInteger function intends to determine whether the input string is a positive integer. However, it incorrectly recognizes "-1" as a positive integer.

The problem lies in the structure of regular expression /[1-9]\d*$/ . It matches:

• [1-9] : A number between 1 and 9.
• \d* : Zero or more numbers (0-9).
• $ : end of string.

When "-1" is entered, the regular expression only matches "1" at the end of the string, satisfying the [1-9] condition, so it returns true . It ignores the "-" symbol at the beginning of the string.

To correctly match positive integers, we need to make sure that the regular expressions match from the beginning of the string and do not contain a negative sign. This can be done by adding the ^ symbol at the beginning of the regular expression, which means that the match must start from the ^ position of the string. The complete corrected regular expression should be: ^[1-9]\d*$

Corrected code:

 function isPositiveInteger(str) {
    const regex = /^[1-9]\d*$/;
    return regex.test(str);
}

console.log(isPositiveInteger("-1")); // Output false
console.log(isPositiveInteger("1")); // Output true
console.log(isPositiveInteger("123")); // Output true
console.log(isPositiveInteger("0")); // Output false

Now, isPositiveInteger function can accurately judge positive integers, avoiding previous incorrect matches. The key is to understand the role of ^ and $ anchors in regular expressions and their importance in ensuring the accuracy of matching positions.

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