为什么边界在正则表达式括号中不起作用？

Question

To begin, I would like to note that a similar question exists with answers and workarounds specific to PHP. I am seeing this issue in C# and I would like to understand the logic behind this apparent "gotcha".

The word boundary character \\b doesn't seem to work properly when placed inside a Regex set (aka "box brackets": [] ). Is this a syntactic issue, are word boundaries intentionally excluded from set matching, or is there some other explanation I'm missing?

Here is a program to demonstrate the issue:

namespace TestProgram
{
    using System.Text.RegularExpressions;
    using System.Diagnostics;
    class Program
    {
        static void Main(string[] args)
        {
            var text = "[abc]";
            var BaselineRegex = new Regex(@"(?:\b)(abc)");
            Debug.Assert(BaselineRegex.IsMatch(text)); // Assertion Passes
            var BracketRegex = new Regex(@"(?:[\b])(abc)");
            Debug.Assert(BracketRegex.IsMatch(text)); // Assertion Fails!
        }
    }
}

Here are web versions to demonstrate as well:

• Word boundary performing as expected without brackets: ( link )

• Word boundary failing to match when placed inside brackets: ( link )

Answer 1

To quote Wiktor Stribiżew's comment :

[\\b] is a backspace char matching pattern, that is all.

So while \\b is a zero-width word boundary outside of a character class, it refers to the backspace character ( 0x8 in ASCII) when used within a character class. Further details are provided in this post .

Wiktor: If you would like to post your own answer I would be happy to accept it over this one.