.NET Regex Error: [x-y] range in reverse order

Question

I am creating a Regex and so far I did this and tried it,

 ^([0][1-9]|1[0-2])[/-.]

and I get the following error.

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Answer 1

Not a bug. Inside a character class (denoted by […]) the - character must be first (some flavours allow first or last, I believe) if it is to be included as a literal. Otherwise it is expected to denote a range, such as 0-9 or A-Z or even /-..

The problem is that according to Unicode, the . comes before the /, so the range is interpreted to be backward, equivalent to specifying a range 7-4.

If you used [.-/], I would not expect a parse exception, but you wouldn't get the results you expected.

Answer 2

The problem is with this part:

[/-.]

That means "the range of characters from '/' to '.'" - but '/' comes after '.' in Unicode, so the range makes no sense.

If you wanted it to mean "slash, dash or period" then you want:

[/\-.]

... in other words, you need to escape the dash. Note that if this is in a regular C# string literal, you'll need to perform another level of escaping too:

string pattern = "[/\\-.]";

Using a verbatim string literal means you don't need to escape the backslash:

string pattern = @"[/\-.]";

Alternatively, as Jay suggested, you can just put the dash at the start:

[-/.]

or end:

[/.-]

(I've just tested, and all three of these options work.)

Answer 3

Inside a character class i.e. [] the - denotes a range, i.e. all lower case letters between a and z can be expressed as [a-z].

What are the range for [/-.]?