Chapter 6. Numbers
Regular expressions are designed to deal with text, and don’t
understand the numerical meanings that humans assign to strings of digits.
To a regular expression,
56 is not the number fifty-six, but a string
consisting of two characters displayed as the digits 5 and 6. The regex
engine knows they’re digits, because the shorthand character class
them (see Recipe 2.3). But that’s it. It doesn’t
56 has a
higher meaning, just as it doesn’t know that
:-) is anything but three punctuation characters
matched by ‹
But numbers are some of the most important input you’re likely to deal with, and sometimes you need to process them inside a regular expression instead of just passing them to a conventional programming language when you want to answer questions such as, “Is this number within the range 1 through 100?” So we’ve devoted a whole chapter to matching all kinds of numbers with regular expressions. We start off with a few recipes that may seem trivial, but actually explain important basic concepts. The later recipes that deal with more complicated regexes assume you grasp these basic concepts.