Regular Expression Question

[Ron Rohrssen]

Slightly off topic....

How can I write a regex that limits user input to 3 digits in the range of
1-128?

I've been trying \d{1,128} but this allows for a match on more than 3
digits.

Thanks.

 

[Jon Skeet [C# MVP]]

How can I write a regex that limits user input to 3 digits in the range of
1-128?

I've been trying \d{1,128} but this allows for a match on more than 3
digits.

Yes - that allows up to 128 digits.

If you want to limit it to 0-128, I suggest you write a simple method
that checks that (e.g. by first checking the length and that all the
characters are digits, and then using Int32.Parse). Is there some
reason you want to use a regular expression?

 

[Daniel Billingsley]

Checking the length and then for a max value of 128 is redundant, isn't it?

 

[Jon Skeet [C# MVP]]

Checking the length and then for a max value of 128 is redundant, isn't it?

Only sort of. It's not actually redundant in that it prevents strings
of the form "0123". It's also usful to do as a "first" check before the
more expensive checks (checking that each character is a digit and the
parsing itself).

 

[Ron Rohrssen]

I can see that I didn't make this clear.

I'm trying to limit the user to 3 digits with a possible range of values
from 1-128.

Not 128 digits.

I'm doing it this way because the edit regex is exposed to the user through
configuration choices. So, I'll restrict them to valid regex expressions.
Later, I'll have to use this expression to validate input by another user.

I've come up with this regex,
\b(12[0-8] | 1[0-1][0-9] | [01][0-2][0-8] | [0-9][0-9] | [0-9])\b

This seems to be ok. Is there a better way?

Ron

 

[Jon Skeet [C# MVP]]

I can see that I didn't make this clear.

I'm trying to limit the user to 3 digits with a possible range of values
from 1-128.

Not 128 digits.

Indeed. The problem was that your regular expression *was* allowing 128
digits, because the \d{x,y} bit basically says "I want between x and y
digits inclusive".

I'm doing it this way because the edit regex is exposed to the user through
configuration choices. So, I'll restrict them to valid regex expressions.
Later, I'll have to use this expression to validate input by another user.

I've come up with this regex,
\b(12[0-8] | 1[0-1][0-9] | [01][0-2][0-8] | [0-9][0-9] | [0-9])\b

This seems to be ok. Is there a better way?

Well, you can make it slightly better:

\b(12[0-8] | 1[0-1][0-9] | 0?[0-9]?[0-9])\b

I *think* that works okay, but to be honest I haven't tried it.

Currently your regular expression isn't terribly clear because the
third "chunk" of it overlaps with the second chunk. (IMO, anyway.)

 

[Ron Rohrssen]

Yes, I see the overlap now. Thanks.

\b(12[0-8] | 1[0-1][0-9] | 0?[0-9]?[0-9])\b

What does the ? mean in your regex?

Ron

I can see that I didn't make this clear.

I'm trying to limit the user to 3 digits with a possible range of values
from 1-128.

Not 128 digits.

Indeed. The problem was that your regular expression *was* allowing 128
digits, because the \d{x,y} bit basically says "I want between x and y
digits inclusive".

I'm doing it this way because the edit regex is exposed to the user through
configuration choices. So, I'll restrict them to valid regex expressions.
Later, I'll have to use this expression to validate input by another user.

I've come up with this regex,
\b(12[0-8] | 1[0-1][0-9] | [01][0-2][0-8] | [0-9][0-9] | [0-9])\b

This seems to be ok. Is there a better way?

Well, you can make it slightly better:

\b(12[0-8] | 1[0-1][0-9] | 0?[0-9]?[0-9])\b

I *think* that works okay, but to be honest I haven't tried it.

Currently your regular expression isn't terribly clear because the
third "chunk" of it overlaps with the second chunk. (IMO, anyway.)

 

[Jon Skeet [C# MVP]]

Yes, I see the overlap now. Thanks.

\b(12[0-8] | 1[0-1][0-9] | 0?[0-9]?[0-9])\b

What does the ? mean in your regex?

It's basically "the preceding character is optional".

I really wouldn't start messing around with regular expressions like
this without reading a thorough tutorial on it and having MSDN handy,
personally.

 

[Ron Rohrssen]

Agreed!

I use regular expressions about once every 2 years. I still haven't found a
straight forward tutorial that doesn't take days to read through. I even
bought the O'Reilly Regular Expressions book when I was doing some Perl
several years ago. At the time I found it pretty useful. But like most
things, if you don't use it, you lose it.

Do you have any "favorite" sites or books that you like to use for
reference?

Ron

Yes, I see the overlap now. Thanks.

\b(12[0-8] | 1[0-1][0-9] | 0?[0-9]?[0-9])\b

What does the ? mean in your regex?

It's basically "the preceding character is optional".

I really wouldn't start messing around with regular expressions like
this without reading a thorough tutorial on it and having MSDN handy,
personally.

 

[Jon Skeet [C# MVP]]

Agreed!

I use regular expressions about once every 2 years. I still haven't found a
straight forward tutorial that doesn't take days to read through. I even
bought the O'Reilly Regular Expressions book when I was doing some Perl
several years ago. At the time I found it pretty useful. But like most
things, if you don't use it, you lose it.
Yup.

Do you have any "favorite" sites or books that you like to use for
reference?

I personally just use the MSDN for the most part - but then, I haven't
had to use regular expressions much outside responding to newsgroup
posts

 

[Daniel Billingsley]

I also found a tool called "The Regex Coach" which was invaluable. It has a
feature to "step" through the evaluation so you can see exactly when things
match or go haywire. It's a standard regex tool and doesn't support all the
features of the .NET implementation.

Ron,

A couple of thoughts.

This is the sort of expression that lends itself quite well to brute-force
testing. It's pretty easy for you to throw a whole mess of values at it
(even the whole Int32 range) to see how it behaves.

My general approach is not to do this kind of thing in regex, as it's so
tough to read and debug. It's much easier to do it in code.

If you do, here's my advice:

1) Download my regex tool from here:

http://www.gotdotnet.com/Community/...mpleGuid=C712F2DF-B026-4D58-8961-4EE2729D7322

2) Write your regexes like:

string pattern = @"
(
\d | # any single digit number
\d\d | # any two-digit number
1[01]\d # 10d or 11d
12[0-8] # 120 - 128
)
"

You'll need to use RegexOptions.IgnorePatternWhitespace

--
Eric Gunnerson

Visit the C# product team at http://www.csharp.net
Eric's blog is at http://weblogs.asp.net/ericgu/

This posting is provided "AS IS" with no warranties, and confers no rights.

Agreed!

I use regular expressions about once every 2 years. I still haven't

found

a
straight forward tutorial that doesn't take days to read through. I even
bought the O'Reilly Regular Expressions book when I was doing some Perl
several years ago. At the time I found it pretty useful. But like most
things, if you don't use it, you lose it.

Do you have any "favorite" sites or books that you like to use for
reference?

Ron

Yes, I see the overlap now. Thanks.

\b(12[0-8] | 1[0-1][0-9] | 0?[0-9]?[0-9])\b

What does the ? mean in your regex?

It's basically "the preceding character is optional".

I really wouldn't start messing around with regular expressions like
this without reading a thorough tutorial on it and having MSDN handy,
personally.