Regular Expressions Cookbook, 2nd Edition

6.5. Decimal Numbers

Problem

You want to find various kinds of integer decimal numbers in a larger body of text, or check whether a string variable holds an integer decimal number. The number must not have a leading zero, as only octal numbers can have leading zeros. But the number zero itself is a valid decimal number.

Solution

Find any positive integer decimal number without a leading zero in a larger body of text:

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

Regex options: None

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

Check whether a text string holds just a positive integer decimal number without a leading zero:

\A(0|[1-9][0-9]*)\Z

Regex options: None

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

^(0|[1-9][0-9]*)$

Regex options: None

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

Discussion

Recipe 6.1 shows a lot of solutions for matching integer decimal numbers, along with a detailed explanation. But the solutions in that recipe do not take into account that in many programming languages, numbers with a leading zero are octal numbers rather than decimal numbers. They simply use ‹[0-9]+› to match any sequence of decimal digits.

The solutions in this recipe exclude numbers with a leading zero, while still matching the number zero itself. Instead of matching any sequence of decimal digits with ‹[0-9]+›, these regular expressions use ‹0|[1-9][0-9]*› to match either the digit zero, or a decimal number with at least one digit that does not begin with a zero. Since the alternation ...