Regular Expressions Cookbook

2.12. Repeat Part of the Regex a Certain Number of Times

Problem

Create regular expressions that match the following kinds of numbers:

• A googol (a decimal number with 100 digits).

• A 32-bit hexadecimal number.

• A 32-bit hexadecimal number with an optional h suffix.

• A floating-point number with an optional integer part, a mandatory fractional part, and an optional exponent. Each part allows any number of digits.

Solution

Googol

\b\d{100}\b

Regex options: None

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

Hexadecimal number

\b[a-z0-9]{1,8}\b

Regex options: Case insensitive

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

Hexadecimal number

\b[a-z0-9]{1,8}h?\b

Regex options: Case insensitive

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

Floating-point number

\d*\.\d+(e\d+)?

Regex options: Case insensitive

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

Discussion

Fixed repetition

The quantifier ‹{n}›, where n is a positive number, repeats the preceding regex token n number of times. The ‹\d{100}› in ‹\b\d{100}\b› matches a string of 100 digits. You could achieve the same by typing ‹\d› 100 times.

‹{1}› repeats the preceding token once, as it would without any quantifier. ‹ab{1}c› is the same regex as ‹abc›.

‹{0}› repeats the preceding token zero times, essentially deleting it from the regular expression. ‹ab{0}c› is the same regex as ‹ac›.

Variable repetition

For variable repetition, we use the quantifier ‹{n,m}›, where n is a positive number and ...