The Personal Web Pages of Chris X. Edwards
Regular Expression Tutorial
It is often desirable to search for a character that matches a character in an ordered sequence. For example, the character class [ABCDEFG] specifies a match for anything between (and including) A and G. There is a short cut for specifying such classes called the range metacharacter which is the "-", the dash.
Here are some examples:
Note that ranges work in negated character classes too. A negated range will match anything not in the range.
Here are some negated examples:
It is easy to see how to construct ranges out of characters with a well known order. We know that "G" comes before "T" because we (correctly) assume that the letters are in alphabetical order. A good question is how are the rest of the characters ordered? For that matter, what is the order relationship between upper and lower case? In some cases, this depends a lot on the character encoding you are using. Regular expression work in Japanese and Hebrew as well as English, but the order of characters that ranges specify will be affected. Generally, it's not a good idea to specify ranges that include characters which don't have a natural order. So if you aren't sure if a "#" comes before a "?" then it's probably a bad idea to use them in a range. If this advice seems overly cautious, you can try using this list to order various characters:
For example, [A-z] includes the characters "[", "/", "]", "^", "_", and "`", as well as all of the upper and lower case letters. It is also apparent that the range [a-Z] is an error because "Z" comes before "a".
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|Chris X. Edwards ~ December 2003|