regular expression(Regex)

  • Thread starter Thread starter Tony Johansson
  • Start date Start date
T

Tony Johansson

Hi!

I just wonder when is it better to use regular(Regex) expression then to use
substring ?

//Tony
 
Tony said:
Hi!

I just wonder when is it better to use regular(Regex) expression then to use
substring ?
Click to expand...

The only time it's better to use a regular expression than substring is
when substring isn't applicable--that is, when what you're trying to
match isn't a fixed string. You wouldn't bother with a regular
expression just to find out if a string contains the substring "foo"
somewhere inside it.
 
Harlan Messinger said:
The only time it's better to use a regular expression than substring is
when substring isn't applicable--that is, when what you're trying to match
isn't a fixed string. You wouldn't bother with a regular expression just
to find out if a string contains the substring "foo" somewhere inside it.
Click to expand...

If I just ask in general when should I use regular expression instead of
doing the same thing with classes

//Tony
 
Tony said:
If I just ask in general when should I use regular expression instead of
doing the same thing with classes
Click to expand...

Then I'm afraid I don't understand the question. Regular expressions are
a way to express a character-based pattern. They're used to confirm that
a string matches, or to locate strings that match, a pattern (assuming
the pattern *can* be expressed as a regular expression--not all string
patterns can). What do you mean by doing "the same thing" with classes?
 
I just wonder when is it better to use regular(Regex) expression then to use
substring ?
Click to expand...

Your goal is readable code.

So as the complexity of the parsing goes up then you change
tool:
IndexOf+Substring -> Regex -> generated Parser

Arne
 
Harlan Messinger said:
Then I'm afraid I don't understand the question. Regular expressions are a
way to express a character-based pattern. They're used to confirm that a
string matches, or to locate strings that match, a pattern (assuming the
pattern *can* be expressed as a regular expression--not all string
patterns can). What do you mean by doing "the same thing" with classes?
Click to expand...

In .NET we have a framework that consist of a lot of classes. these classes
can be used when
we want to change the state of an object. So in almost every case whan you
will use a regex you can almost always do the same this by using the classes
that exist in the framework but the code will be greater then using regex.

Hope you understand what I mean.

//Tony
 
In .NET we have a framework that consist of a lot of classes. these classes
can be used when
we want to change the state of an object. So in almost every case whan you
will use a regex you can almost always do the same this by using the classes
that exist in the framework but the code will be greater then using regex.

Hope you understand what I mean.
Click to expand...

Not me.

The Regex class and its associated classes provide some
functionality for text matching.

The String class also have some functionality (IndexOf, Substring etc.)
that can be used for the same.

Regex is not about changing state in arbitrary objects. It is text
tool.

I will attach a few examples below.

Arne

========================

using System;
using System.Text.RegularExpressions;

namespace E
{
public class Program
{
private static Regex re = new
Regex(@"(?:<a[^>]+href\s*=\s*[""'])([^""']*)(?:[""'][^>]*>)([^<]*)(?:</a>)",
RegexOptions.IgnoreCase);
public static void Find(string html)
{
foreach(Match m in re.Matches(html))
{
Console.WriteLine("link=" + m.Groups[1].Value);
Console.WriteLine("title=" + m.Groups[2].Value);
}
}
public static void Main(string[] args)
{
Find("bla <a href='foo.html'>Foo</a> bla <a
href='bar.html'>Bar</a> bla");
}
}
}



using System;
using System.Text.RegularExpressions;

namespace E
{
public class Program
{
private static string parsePhp(string php)
{
return "#" + php + "#";
}
private static readonly Regex re = new
Regex(@"(<\?)(.*?)(\?>)", RegexOptions.Compiled | RegexOptions.Singleline);
public static string Parse(string s)
{
string res = s;
foreach(Match m in re.Matches(s))
{
res = res.Replace(m.Groups[0].Value,
parsePhp(m.Groups[2].Value));
}
return res;
}
public static void Main(string[] args)
{
string s = @"<?
include(""inc/db.php"");
include(""inc/layout.php"");
print_header();
?>
<div>
<b>Dette er noget HTML</b>
</div>
<?
print_footer();
?>";
Console.WriteLine(Parse(s));
}
}
}





using System;
using System.Text.RegularExpressions;

namespace E
{
public class Program
{
private static readonly Regex re = new Regex(@"(\d+)
(\d+)/(\d+)", RegexOptions.Compiled);
public static double FractionParse(string s)
{
Match m = re.Match(s);
if(m.Success)
{
return int.Parse(m.Groups[1].Value) +
int.Parse(m.Groups[2].Value) / (double)int.Parse(m.Groups[3].Value);
}
else
{
throw new ArgumentException(s + " is not a valid
fraction");
}
}
public static void Main(string[] args)
{
string s = "5 1/8";
Console.WriteLine(FractionParse(s));
}
}
}




using System;
using System.Text.RegularExpressions;

namespace E
{
public class Program
{
public static void Main(string[] args)
{
string s = "xxx [kEy1] yyy [KeY2] zzz";
string sx = s;
sx = Regex.Replace(sx, @"\[key1\]", "foo",
RegexOptions.IgnoreCase);
sx = Regex.Replace(sx, @"\[key2\]", "bar",
RegexOptions.IgnoreCase);
Console.WriteLine(sx);
}
}
}
 
Back
Top