Re: RegExp Again!

Hello RN1,

> Using Regular Expression, I want to ensure that users enter either a
> 2- digit or a 3-digit whole number in a TextBox. This is how I framed
> the ValidationExpression in the RegularExpressionValidator:
>
> ------------------------------------------------------------ ----------
> ----------
>
> > Display="dynamic" ErrorMessage="Invalid Text"
> ValidationExpression="[0-9]{3}|[0-9]{2}" runat="server"/>
> ------------------------------------------------------------ ----------
> ----------
> Suppose a user enters 16 & 216 in the TextBox. As expected, both the
> numbers return True but if I just reverse the ValidationExpression
> i.e. change the ValidationExpression from
>
> [0-9]{3}|[0-9]{2}
>
> to
>
> [0-9]{2}|[0-9]{3}
>
> & then input 16 & 216 in the TextBox, 16 correctly evaluates to True
> but 216 strangely evaluates to False!
>
> After a plethora of trial & error methods, I realized that if the
> second ValidationExpression (the one which evaluates 216 to False) is
> enclosed in brackets & a $ sign is appended at the end of the
> expression like this
>
> ([0-9]{2}|[0-9]{3})$
>
> then 216 correctly evaluates to True (& so does 16).
>
> What I couldn't figure out is the logic behind the expression when it
> is wrapped in brackets & a $ sign is appended at the end! Can someone
> please explain me this? What more work does the edited
> ValidationExpression do to make 216 evaluate to True?
>
> I am aware that $ means the end of a string.


To start with, [0-9]{2,3} would be the sorter variant of your original expression,
but to explain why this isn't working as expected: The RegularExpressionValidator
puts a ^ and a $ around every expression, so the expression under test is
actually:

^[0-9]{2}|[0-9]{3}$

Which should be read as:

^[0-9]{2} or [0-9]{3}$

So: any string starting with 2 numbers or any string ending in two numbers.

([0-9]{2}|[0-9]{3})

Adding () solves this because that in turn expands to

^([0-9]{2}|[0-9]{3})$ or ^[0-9]{2}$|^[0-9]{3}$

The best solution is to make sure all your validation expressions are either
in (..), or ^..$.

--
Jesse Houwing
jesse.houwing at sogeti.nl