May 2009
Intermediate to advanced
510 pages
15h
English
Content preview from Regular Expressions Cookbook
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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
hsuffix.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 ‹{›, where n}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 ‹{›, where
n,m}n is a positive number and ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.