Regular Expressions Cookbook

6.8. Numbers with Thousand Separators

Problem

You want to match numbers that use the comma as the thousand separator and the dot as the decimal separator.

Solution

Mandatory integer and fraction:

^[0-9]{1,3}(,[0-9]{3})*\.[0-9]+$

Regex options: None

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

Mandatory integer and optional fraction. Decimal dot must be omitted if the fraction is omitted.

^[0-9]{1,3}(,[0-9]{3})*(\.[0-9]+)?$

Regex options: None

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

Optional integer and optional fraction. Decimal dot must be omitted if the fraction is omitted.

^([0-9]{1,3}(,[0-9]{3})*(\.[0-9]+)?|\.[0-9]+)$

Regex options: None

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

The preceding regex, edited to find the number in a larger body of text:

\b[0-9]{1,3}(,[0-9]{3})*(\.[0-9]+)?\b|\.[0-9]+\b

Regex options: None

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

Discussion

Since these are all regular expressions for matching floating-point numbers, they use the same techniques as the previous recipe. The only difference is that instead of simply matching the integer part with ‹[0-9]+›, we now use ‹[0-9]{1,3}(,[0-9]{3})*›. This regular expression matches between 1 and 3 digits, followed by zero or more groups that consist of a comma and 3 digits.

We cannot use ‹[0-9]{0,3}(,[0-9]{3})*› to make the integer part optional, because that would match numbers with a leading comma, e.g., ,123. It’s the same trap of making ...