Regex Usage, or use Substring?

**Jon Skeet [C# MVP]** · Jun 27 '08, 07:59 PM

Re: Regex Usage, or use Substring?

jp2msft <jp2msft@discus sions.microsoft .comwrote:

Two part question:
>
1. Is Regex more efficient than manually comparing values using Substring?

Sometimes.

2. I've never created a Regex expression. How would I use regex to do the
equivalent of what I have coded using Substrings below?

Well, let's start off by getting rid of the unnecessary substrings,
when you're only looking at a single character:

string s = TextBox1.Text.T oUpper();
char ch = s[0]; // read first character [B,C,X]
ok = false; // Only one path can produce ok = true
if (ch=='B' || ch=='C' || ch=='X')
{
ch = s[1]; // read second character [P,0,5,7]
if (ch == 'P' || ch == '0' || ch == '5' || ch == '7'))
{
if (len 12) // Reversed if (12 < len) for readability
{
if (s[7] == ' ' && s[12] == ' ')
{
if (len == 15)
{
ok = IsNumeric(new string[] { s.Substring(2, 5),
s.Substring(8, 4),
s.Substring(13, 2) });
}
else
{
// This message looks incorrect - we've just checked
// the length, not the data
Console.WriteLi ne
("Non-numeric data found in numeric section");
}
}
else
{
Console.WriteLi ne
("There should be blanks at positions 8 and 13.");
}
}
}
else
{
Console.WriteLi ne("Second letter should be [P, 0, 5, or 7].");
}
}
else
{
Console.WriteLi ne("First letter should be [B, C, or X].");
}

Okay, now assuming that the IsNumeric call is actually to check whether
all of those characters are digits, we could indeed convert all of this
into a regular expression - but it would either match or not. It
wouldn't say which part failed. The regex would be something like:

@"^[BCH][P057]\d\d\d\d\d \d\d\d\d \d\d$"

I haven't tested it, but it looks okay to inspection...

--
Jon Skeet - <skeet@pobox.co m>
Web site: http://www.pobox.com/~skeet
Blog: http://www.msmvps.com/jon_skeet
C# in Depth: http://csharpindepth.com

**=?UTF-8?B?QXJuZSBWYWpow7hq?=** · Jun 27 '08, 07:59 PM

Re: Regex Usage, or use Substring?

jp2msft wrote:

Two part question:
>
1. Is Regex more efficient than manually comparing values using Substring?

Only if you write the manual comparing code poorly.

2. I've never created a Regex expression. How would I use regex to do the
equivalent of what I have coded using Substrings below?

Code:

string s = TextBox1.Text.ToUpper();
string ch = s.Substring(0, 1); // read first character [B,C,X]
if ((ch == "B") || (ch == "C") || (ch == "X")) {
  ch = s.Substring(1, 1); // read second character [P,0,5,7]
  if ((ch == "P") || (ch == "0") || (ch == "5") || (ch == "7")) {
    if (12 < len) {
      ch = " "; // check for spaces in 2 places
      if ((s.Substring(7, 1) == ch) && (s.Substring(12, 1) == ch)) {
        if (len == 15) {
          ok = IsNumeric(new string[] { s.Substring(2, 5), s.Substring(8,
4), s.Substring(13, 2) });
          } else {
            Console.WriteLine("Non-numeric data found in numeric section.");
          }
        }
      } else {
        Console.WriteLine("There should be blanks at positions 8 and 13.");
        ok = false;
      }
    }
  } else {
    Console.WriteLine("Second letter should be [P, 0, 5, or 7].");
    ok = false;
  }
} else {
  Console.WriteLine("First letter should be [B, C, or X].");
  ok = false;
}

This is a good example of something that should be done as regex,
because the regex will be much easier to read and maintain.

"[BCX][P057]\d{5} \d{4} \d{2}"

should work.

Arne

**Mythran** · Jun 27 '08, 07:59 PM

Re: Regex Usage, or use Substring?

"Jon Skeet [C# MVP]" <skeet@pobox.co mwrote in message
news:MPG.22ccbd 24b62dc685db9@m snews.microsoft .com...

jp2msft <jp2msft@discus sions.microsoft .comwrote:

>Two part question:
>>
>1. Is Regex more efficient than manually comparing values using
>Substring?

>
Sometimes.
>

>2. I've never created a Regex expression. How would I use regex to do the
>equivalent of what I have coded using Substrings below?

>
Well, let's start off by getting rid of the unnecessary substrings,
when you're only looking at a single character:
>
string s = TextBox1.Text.T oUpper();
char ch = s[0]; // read first character [B,C,X]
ok = false; // Only one path can produce ok = true
if (ch=='B' || ch=='C' || ch=='X')
{
ch = s[1]; // read second character [P,0,5,7]
if (ch == 'P' || ch == '0' || ch == '5' || ch == '7'))
{
if (len 12) // Reversed if (12 < len) for readability
{
if (s[7] == ' ' && s[12] == ' ')
{
if (len == 15)
{
ok = IsNumeric(new string[] { s.Substring(2, 5),
s.Substring(8, 4),
s.Substring(13, 2) });
}
else
{
// This message looks incorrect - we've just checked
// the length, not the data
Console.WriteLi ne
("Non-numeric data found in numeric section");
}
}
else
{
Console.WriteLi ne
("There should be blanks at positions 8 and 13.");
}
}
}
else
{
Console.WriteLi ne("Second letter should be [P, 0, 5, or 7].");
}
}
else
{
Console.WriteLi ne("First letter should be [B, C, or X].");
}
>
Okay, now assuming that the IsNumeric call is actually to check whether
all of those characters are digits, we could indeed convert all of this
into a regular expression - but it would either match or not. It
wouldn't say which part failed. The regex would be something like:
>
@"^[BCH][P057]\d\d\d\d\d \d\d\d\d \d\d$"
>
I haven't tested it, but it looks okay to inspection...
>

Jon, I believe the first character should be B, C, or X, not H... other
than that, looks ok to me :)

Mythran

Regex Usage, or use Substring?

Regex Usage, or use Substring?

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