Code
using System;
using System.Text.RegularExpressions;
namespace RegexNoMatch {
class Program {
static void Main () {
string input = "a foobar& b";
string regex1 = "(foobar|foo)&?";
string regex2 = "(foo|foobar)&?";
string replace = "$1";
Console.WriteLine(Regex.Replace(input, regex1, replace));
Console.WriteLine(Regex.Replace(input, regex2, replace));
Console.ReadKey();
}
}
}
Expected output
a foobar b
a foobar b
Actual output
a foobar b
a foobar& b
Question
Why does replacing not work when the order of "foo" and "foobar" in regex pattern is changed? How to fix this?
The regular expression engine tries to match the alternatives in the order in which they are specified. So when the pattern is (foo|foobar)&? it matches foo immediately and continues trying to find matches. The next bit of the input string is bar& b which cannot be matched.
In other words, because foo is part of foobar, there is no way (foo|foobar) will ever match foobar, since it will always match foo first.
때때로 이것은 실제로 매우 유용한 트릭이 될 수 있습니다. 패턴은 (o|a|(\w))캡처 할 수 \w및 a또는 o다른 :
Regex.Replace("a foobar& b", "(o|a|(\\w))", "$2") // fbr& b
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