RegEx for a string of length 0-2

Question

I'm trying to match a string that can be either empty or have 1 or 2 numbers in it such as the following:

"" (empty) "1" "23"

String with more numbers or non-numeric characters should not match. My closest guess is the regex:

[0-9]{0,2}

Which I read to say "the numbers 0 through 9 occurring 0 to 2 times." However, in practice I find that regex also matches longer strings like "333". How is it possible to restrict string length in regular expressions?

Answer 1

Use the following regex:

^[0-9]{0,2}$ 

You almost had it -- the ^ and $ characters are anchors that match the beginning and end of the string, respectively.

For a more in-depth discussion on anchors, see here:

[Anchors] do not match any character at all. Instead, they match a position before, after or between characters. They can be used to "anchor" the regex match at a certain position.

Answer 2

You need to anchor the regex:

^[0-9]{0,2}$ 

Otherwise the regex will happily match substrings.