Detail on a regex?

Ruby
1

Is there a way to pull details out of a regular expression’s failure?

I’m running into a situation where I’ve got some validation methods,
and I’m resorting to passing in both a regular expression and a range.
I know quite well I could just use a range inside of a regex, and
would rather, but I need to get the range in order to have some useful
error messages.

For example, the string must be 3 to 8 characters long, start with a
letter, and may contain letters numbers or dashes.

I would like to just use this:
some_string=~/^[a-z][-a-z0-9]{2,7}$/i

But if the match fails, I can’t give out any good error messages
without further inspecting the string, so I end up with
some_string=~/^[a-z][-a-z0-9]$/i
[3…8].include?some_string.length

Just so I can give reasonable error messages.

–Kyle

2

On 13.05.2008 17:59, Kyle S. wrote:

I would like to just use this:
some_string=~/^[a-z][-a-z0-9]{2,7}$/i

But if the match fails, I can’t give out any good error messages
without further inspecting the string, so I end up with
some_string=~/^[a-z][-a-z0-9]$/i
[3…8].include?some_string.length

Just so I can give reasonable error messages.

If it does not match you get no information out of the regexp. The
simplest would be to just include the string in an error message. To
avoid ugly errors with lengthy strings you could add a length check
before the regexp match.

In some cases it might be possible to build the regexp so that it
matches chunks:

some_string=~/^([a-z])?([-a-z0-9]{2,7})?(.*)$/i

Now you have a match if $1 and $2 are non nil and $3 is empty.

HTH

robert

3

Hum. Yea, but I want the error messages to be helpful for when I’m
not the one running the program.
And also somewhat unique to the reason the error was thrown, just in
case I need to track it down and find out there’s a logic error on my
part.

Not that that ever happens…

On Tue, May 13, 2008 at 11:45 AM, Robert K.

4

Please do not top post.

On 13.05.2008 23:32, Kyle S. wrote:

Hum. Yea, but I want the error messages to be helpful for when I’m
not the one running the program.

Usually I’d make the error message contain a description of the valid
format in prose or sufficient examples if I feel that users might not
know this. An alternative is to dump command line help if there is a
parameter error.

And also somewhat unique to the reason the error was thrown, just in
case I need to track it down and find out there’s a logic error on my
part.

For that you sure have unit tests in place… :slight_smile:

Kind regards

robert