Regular Expressions Cookbook, 2nd Edition

6.6. Strip Leading Zeros

Problem

You want to match an integer number, and either return the number without any leading zeros or delete the leading zeros.

Solution

Regular expression

\b0*([1-9][0-9]*|0)\b

Regex options: None

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Replacement

$1

Replacement text flavors: .NET, Java, JavaScript, PHP, Perl

\1

Replacement text flavors: PHP, Python, Ruby

Getting the numbers in Perl

while ($subject =~ m/\b0*([1-9][0-9]*|0)\b/g) {
    push(@list, $1);
}

Stripping leading zeros in PHP

$result = preg_replace('/\b0*([1-9][0-9]*|0)\b/', '$1', $subject);

Discussion

We use a capturing group to separate a number from its leading zeros. Before the group, ‹0*› matches the leading zeros, if any. Within the group, ‹[1-9][0-9]*› matches a number that consists of one or more digits, with the first digit being nonzero. The number can begin with a zero only if the number is zero itself. The word boundaries make sure we don’t match partial numbers, as explained in Recipe 6.1.

To get a list of all numbers in the subject text without leading zeros, iterate over the regex matches as explained in Recipe 3.11. Inside the loop, retrieve the text matched by the first (and only) capturing group, as explained in Recipe 3.9. The solution for this shows how you could do this in Perl.

Stripping the leading zeros is easy with a search-and-replace. Our regex has a capturing group that separates the number from its leading zeros. If we replace the overall regex match (the number including ...