private static string ReplaceEx(string original,
string pattern, string replacement)
{
int count, position0, position1;
count = position0 = position1 = 0;
string upperString = original.ToUpper();
string upperPattern = pattern.ToUpper();
int inc = (original.Length/pattern.Length) *
(replacement.Length-pattern.Length);
char [] chars = new char[original.Length + Math.Max(0, inc)];
while( (position1 = upperString.IndexOf(upperPattern,
position0)) != -1 )
{
for ( int i=position0 ; i < position1 ; ++i )
chars[count++] = original[i];
for ( int i=0 ; i < replacement.Length ; ++i )
chars[count++] = replacement[i];
position0 = position1+pattern.Length;
}
if ( position0 == 0 ) return original;
for ( int i=position0 ; i < original.Length ; ++i )
chars[count++] = original[i];
return new string(chars, 0, count);
}

测试

static void Main(string[] args)
{
string segment = "AaBbCc";
string source;
string pattern = "AbC";
string destination = "Some";
string result = "";

const long count = 1000;
StringBuilder pressure = new StringBuilder();
HiPerfTimer time;

for (int i = 0; i < count; i++)
{
pressure.Append(segment);
}
source = pressure.ToString();
GC.Collect();

//regexp
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = Regex.Replace(source, pattern,
destination, RegexOptions.IgnoreCase);
}
time.Stop();

Console.WriteLine("regexp = " + time.Duration + "s");
GC.Collect();

//vb
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = Strings.Replace(source, pattern,
destination, 1, -1, CompareMethod.Text);
}
time.Stop();

Console.WriteLine("vb = " + time.Duration + "s");
GC.Collect();


//vbReplace
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = VBString.Replace(source, pattern,
destination, 1, -1, StringCompareMethod.Text);
}
time.Stop();

Console.WriteLine("vbReplace = " + time.Duration + "s");// + result);
GC.Collect();


// ReplaceEx
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = Test.ReplaceEx(source, pattern, destination);
}
time.Stop();

Console.WriteLine("ReplaceEx = " + time.Duration + "s");
GC.Collect();


// Replace
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = source.Replace(pattern.ToLower(), destination);
}
time.Stop();

Console.WriteLine("Replace = " + time.Duration + "s");
GC.Collect();


//sorry, two slow :(
/*//substring
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = StringHelper.ReplaceText(source, pattern,
destination, StringHelper.CompareMethods.Text);
}
time.Stop();

Console.WriteLine("substring =" + time.Duration + ":");
GC.Collect();


//substring with stringbuilder
time = new HiPerfTimer();
time.Start();
for (int i = 0; i < count; i++)
{
result = StringHelper.ReplaceTextB(source, pattern,
destination, StringHelper.CompareMethods.Text);
}
time.Stop();

Console.WriteLine("substringB=" + time.Duration + ":");
GC.Collect();
*/

Console.ReadLine();
}

1¡¢string segment = "abcaBc";
regexp = 3.75481827997692s
vb = 1.52745502570857s
vbReplace = 1.46234256029747s
ReplaceEx = 0.797071415501132s !!!Replace = 0.178327413120941s
// ReplaceEx > vbReplace > vb > regexp

2¡¢string segment = "abcaBcabC";
regexp = 5.30117431126023s
vb = 2.46258449048692s
vbReplace = 2.5018721653171s
ReplaceEx = 1.00662179131705s !!!
Replace = 0.233760994763301s
// ReplaceEx > vb > vbReplace > regexp

3¡¢string segment = "abcaBcabCAbc";
regexp = 7.00987862982586s
vb = 3.61050301085753s
vbReplace = 3.42324876485699s
ReplaceEx = 1.14969947297771s !!!
Replace = 0.277254511397398s
// ReplaceEx > vbReplace > vb > regexp

4¡¢string segment = "ABCabcAbCaBcAbcabCABCAbcaBC";
regexp = 13.5940090151123s
vb = 11.6806222578568s
vbReplace = 11.1757614445411s
ReplaceEx = 1.70264153684337s !!!(my god!)
Replace = 0.42236820601501s
// ReplaceEx > vbReplace > vb > regexp

查看程序的Block在：

string upperString = original.ToUpper();
string upperPattern = pattern.ToUpper();

如果需要敏感，就免了这2行。

解释：先建一个char[]类型的变量采访替换后的字符，其大小就是最大可能被替换的字符，例如ABABAB，替换AB成C，其获取过程就是ABABAB最大可能包括的AB的数目乘以AB多于C的数目，

char [] chars = new char[original.Length + Math.Max(0, inc)];