Re: any regex gurus out there?
From: Niki Estner (niki.estner_at_cube.net)
Date: 09/22/04
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Date: Wed, 22 Sep 2004 14:38:29 +0200
"bill tie" <billtie@discussions.microsoft.com> wrote in
news:DDA274DF-88D1-47C6-9B36-61D328439DF5@microsoft.com...
> ...
> myString = "foo & ! ( \ * bar";
> // after we clean up we should have only "foo spaces bar"
> myString = Regex.Replace( myString, "[\\\!&*(]", "" );
>
> No matter what number or combination of backslashes I used it didn't work.
I assume the first string constant really is myString = "foo & ! ( \\ *
bar"; - The compiler wouldn't take it otherwise.
The pattern you want to pass to the regex class looks like this: [\\!&*(]
You can either write it this way:
myString = Regex.Replace( myString, @"[\\!&*(]", "" );
or escape each backslash with a backslash, like this:
myString = Regex.Replace( myString, "[\\\\!&*(]", "" );
Note that this is exactly the same code, it's just a different way to write
the string constant.
Also, this doesn't have to do anything with regular expressions: If you want
to code a UNC-path in your code like \\SomeMachine\SomeFolder, you'd either
write @"\\SomeMachine\SomeFolder", or "\\\\SomeMachine\\SomeFolder".
> The point of this exercise is I want to remove everything except:
> letters a through z, A through Z
> numerals 0 through 9
> hyphen -
> space " "
>
> I thought I could do it by negation as follows:
>
> myString = Regex.Replace( myString, "[^0-9a-zA-Z ]", "" );
>
> This would be fantastic. There seem to be two problems:
>
> (a) Regex considers some characters, e.g. parentheses, as letters.
I don't think so. Maybe you have a sample?
> {b) Letters the user enters may not be ordinary English letters. In the
> French language, the letter A may have three different accents. I'm not
> sure
> how the pattern [a-z] treats these letters.
They're not included in this set. You'll have to use unicode character
classes for that problem:
myString = Regex.Replace( myString, @"[^\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Nd} ]",
"" );
(As usual, if you want a non-verbatim string, replace all \ characters with
\\)
\p matches for a unicode character class:
\p{Ll}matches for "Letter lowercase", no matter what language
\p{Lu}matches for "Letter uppercase", no matter what language
\p{Lt}matches for "Letter titlecase", no matter what language
\p{Lo}matches for "other Letters", no matter what language
\p{Nd}matches for "Numeric decimal"
A list of all unicode character classes can be found here:
http://msdn.microsoft.com/library/deu/default.asp?url=/library/DEU/cpref/html/frlrfsystemglobalizationunicodecategoryclasstopic.asp
Note that if you want to include underscores in this set you can use the
shortcut '\w' resp. '\W'.
myString = Regex.Replace( myString, @"[^\w ]", "" );
> [2]
>> The problem with the MatchEvaluator-technique is that
>> calling delegates is quite slow, so 15 compiled regex's
>> might well be faster than one MatchEvaluator-Pass.
>
> I was wondering, too. Yet, some compilers optimize code. I'm terribly
> new
> to C#. I don't know whether it collapses my "calls" into some sort of
> loop
> at compile-time.
There's a good article on .NET optimizations:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/fastmanagedcode.asp
As you can see there, delegates are a lot slower than usual function calls.
>> I'd definitely favour readability over performance.
>
> I concur the readability of the matching algorithm is awful.
Only if you don't know regular expressions. (And that probably applies to
every kind of code)
To use one of your previous examples:
myString = Regex.Replace( myString, "[^0-9a-zA-Z ]", "" );
I think readability of this line is great: Think about what the
corresponding C# code would look like; I'm pretty sure it would be a lot
more complex, and harder to read.
And you can always use tools like Expresso that make work with regex's
really easy.
So, I would disagree in that point.
> Albeit I'm more inclined to apply the classic technique I started with,
> I'd
> like to complete this intellectual exercise. Understanding how matching
> and
> grouping work may be useful in other situations.
>
> Using my last example:
>
> Replace
>
> [abcd] with "z"
> [pqrs] with "y"
> [123] with "x"
>
> string myString = "abcd efg pqrs t 1 k 2 m 3 n";
> myString = Regex.Replace( myString, "(([abcd])|[pqrs])|[123]",
> new MatchEvaluator(MyEvaluator));
Paranthesis is wrong in this pattern. Use this one:
myString = Regex.Replace( myString, "([abcd])|([pqrs])|([123])",
new MatchEvaluator(MyEvaluator));
> ...
> My code looked something like this:
>
> static string MyEvaluator(Match m)
> {
> for ( int i = 0; i < m.Groups.Count; i++ )
> {
> if (m.Groups[i].Length != 0)
> {
> switch (i)
> {
> case 0:
> return "x";
> case 1:
> return "y";
> case 2:
> return "z";
> }
> }
> return null;
> }
> return null;
> }
>
> This didn't do the job I wanted.
Yes, there's a tweak here: Groups[0] always refers to the whole capture;
Groups[1] returns the contents of the first paranthesis, and so on.
Try this one:
static string MyEvaluator(Match m)
{
for ( int i = 1; i < m.Groups.Count; i++ )
{
if (m.Groups[i].Length != 0)
{
switch (i)
{
case 1:
return "x";
case 2:
return "y";
case 3:
return "z";
}
}
}
return null;
}
Niki
- Next message: Ignacio Machin \( .NET/ C# MVP \): "Re: Convert a string containing a 32-bit binary date to a date data type"
- Previous message: Keith Henderson: "Re: non-class methods"
- In reply to: bill tie: "Re: any regex gurus out there?"
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