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I had been using [0-9]{9,12} all along to signify that the numeric string has a length of 9 or 12 characters. However I now realized that it will match input strings of length 10 or 11 as well. So I came out with the naive:
( [0-9]{9} | [0-9]{12} )
Is there a more succinct regex to represent this ?
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The character + in a regular expression means "match the preceding character one or more times". For example A+ matches one or more of character A. The plus character, used in a regular expression, is called a Kleene plus .
Matching 0, 1, or More Occurrences. * matches zero or more occurrences of the preceding character. The fewest possible occurrences of a pattern will satisfy the match. Example: a*b will match b, ab, aab, aaab, aaaab, and so on.
The Match(String, String, RegexOptions) method returns the first substring that matches a regular expression pattern in an input string. For information about the language elements used to build a regular expression pattern, see Regular Expression Language - Quick Reference.
A repeat is an expression that is repeated an arbitrary number of times. An expression followed by '*' can be repeated any number of times, including zero. An expression followed by '+' can be repeated any number of times, but at least once.
You could save one character by using
[0-9]{9}([0-9]{3})?
but in my opinion your way is better because it conveys your intention more clearly. Regexes are hard enough to read already.
Of course you could use \d instead of [0-9].
(Edit: I first thought you could drop the parens around [0-9]{3} but you can't; the question mark will be ignored. So you only save one character, not three.)
(Edit 2: You will also need to anchor the regex with ^ and $ (or \b) or re.match() will also match 123456789 within 1234567890.)
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