c# - 在精确偏移处匹配正则表达式

Question

我想检查某个模式(例如，双引号字符串)是否在确切位置匹配。

示例

string text = "aaabbb";
Regex regex = new Regex("b+");
// Now match regex at exactly char 3 (offset) of text

我想检查 regex 是否正好匹配第 3 个字符。
我查看了 Regex.Match Method (String, Int32) 但它的行为并不像我预期的那样。
所以我做了一些测试和一些解决方法:

public void RegexTest2()
{
    Match m;
    string text = "aaabbb";
    int offset = 3;

    m = new Regex("^a+").Match(text, 0); // lets do a sanity check first
    Assert.AreEqual(true, m.Success);
    Assert.AreEqual("aaa", m.Value);  // works as expected

    m = new Regex("^b+").Match(text, offset);
    Assert.AreEqual(false, m.Success);  // this is quite strange...

    m = new Regex("^.{"+offset+"}(b+)").Match(text); // works, but is not very 'nice'
    Assert.AreEqual(true, m.Success);
    Assert.AreEqual("bbb", m.Groups[1].Value);

    m = new Regex("^b+").Match(text.Substring(offset)); // works too, but 
    Assert.AreEqual(true, m.Success);
    Assert.AreEqual("bbb", m.Value);
}

事实上，我开始相信 new Regex("^.", 1).Match(myString) 永远不会匹配任何东西。

有什么建议吗？

编辑:

我得到了一个可行的解决方案(解决方法)。所以我的问题是关于速度和良好的实现。

Answer 1

你有没有试过docs说？

If you want to restrict a match so that it begins at a particular character position in the string and the regular expression engine does not scan the remainder of the string for a match, anchor the regular expression with a \G (at the left for a left-to-right pattern, or at the right for a right-to-left pattern). This restricts the match so it must start exactly at startat.

即将 ^ 替换为 \G:

m = new Regex(@"\\Gb+").Match(text, offset);
Assert.AreEqual(true, m.Success);  // should now work