Regular expression matching a numerical string exactly - regex

Question

I have a string which looks like this 600/-4.412/11 and one which looks like this 600/11
[optional sign][float or integer]/[optional sign][float or integer]/[optional sign][float or integer]
[optional sign][float or integer]/[optional sign][float or integer]
Example:
1) 600/-4.412/11
2) 600/11
And I need to find a regular expression which matches 1 and one which matches 2. But both expressions mustn't select/match the other one. With my humble regex knowledge I managed to build this expression:
([-+]?[0-9]+(\.?[0-9]+)?\/?){3}
The problem with this expression is that it matches 1) as well as 2) according to http://gskinner.com/RegExr/. Hopefully someone can fix this or at least tell me why this is happening since I hoped that I only had to change {3} to {2} in order to get the different matching.

Answer 1

Problem
The problem is that your regex allows the repeated subpattern, i.e. [-+]?[0-9]+(\.?[0-9]+)?\/? to match without restricting it to each section of numbers which are delimited by /.
For this example in your question: 600/11, the first repetition will match 600/, the second will be 1 and the third one will be the last 1.
Solution 1
WRONG attempt
For validation, you can change it slightly to make it works as you want:
^([-+]?[0-9]+(\.[0-9]+)?(?:/|$)){3}$
(?:/|$) forces a number (floating point or integer) to end with a /, or it is the end of string. This will effectively make sure each repetition will not match within the same number.
^ is added in front and $ behind to make sure that the string has exactly 3 numbers.
The text that are not crossed out still applies in the correct solution.
The CORRECT solution
However, the above regex is WRONG. It will still allow invalid input such as 1/2/3/ to match (ends with /). We need to add an extra assertion at the end to prevent the case above from matching:
^([-+]?[0-9]+(\.[0-9]+)?(?:/|$)){3}(?<!/)$
(?<!/) is a zero-width negative look-behind which checks that the character before the end of the string is not /.
Solution 2
It is less buggy to write the regex in the form [number]([delimiter][number]){repeat} in such cases, rather than fiddling with the form ([number][delimiter/ending]){repeat}.
The answer below will strictly validates the input:
^[-+]?[0-9]+(\.[0-9]+)?(?:/[-+]?[0-9]+(\.[0-9]+)?){2}$
The above is for matching the case of exactly 3 numbers. Change 2 to 1 (or remove {2}) to match exactly 2 numbers.

Answer 2

This happens because almost everything in your expression has been made optional. The only thing that must appear is a single digit, so your expression would also match 007.
It follows that the solution is to make parts of the expression mandatory. There are lots of ways to approach this. One that does not exactly fit your description but which IMHO you should consider nonetheless is
([-+]?[0-9]+(\.[0-9]+)?(?=/|$))
This expression will match both types of inputs, but you can tell them apart by just looking at the count of matches (2 or 3) -- this would be equivalent to testing two different expressions before taking a branch.
Update:
The expression above is too liberal because it is not anchored at the start and end of the input. Here is one that is anchored, and thus will not match if the input contains any spurious characters:
^([-+]?[0-9]+(\.[0-9]+)?(/(?!$)|$)){2,3}$
Breaking it down:
^ start matching at the beginning
( match the following:
[-+]? optional sign
[0-9]+(\.[0-9]+)? integer or float
( either
/(?!$) a slash that's not trailing
| or
$ end of input
)
){2,3} do this exactly two or three times
$ and make sure there is no other input