数字的正则表达式，一些特殊字符和NULL

Question

I want to create a regex which would allow

0-9
.
>
<
%
,
-
NULL

so I wrote a regex like ^[0-9.><%,-NULL]*$

This matches NULL but should not match when user only enters N . So I did ^[0-9.><%,-(NULL)]*$ but I do

string pattern = "^[0-9.><%,-(NULL)]*$";
Regex.Match("N", pattern).Success;

this throws Argument exception and says

`parsing "^[0-9.><%,-(NULL)]*$" - [x-y] range in reverse order.

So what should be the correct Regex?

Answer 1

I think the expression you need looks similar to this:

"^([-0-9.><%,]|NULL)*$"

This will match zero or more occurrences of any of the characters in the character set (inside the square brackets) and instances of NULL .

---

Making NULL part of your character set will also match the N , U and L characters separately. Attempting to group using (NULL) will not have the desired effect, but will additionally make the character set also match the opening and closing parentheses.

The error you mention ( [xy] range in reverse order ) is caused by the fact that in your expression

"^[0-9.><%,-(NULL)]*$"

the hyphen is considered to denote this character range: ,-( . Because the parenthesis precedes the comma, the character range is considered to be in reverse order.

To represent a hyphen ( - ) in a character set, you need to avoid it being interpreted as denoting a character range. This is most easily achieved by placing the dash at the beginning or the end of the character set.