An example Analysis of looking around in reverse order of regular expression matching principle

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I encountered this problem in the CSDN forum the other day:

Var str= "8912341253789"

The duplicate number in this string needs to be removed, which is the result 89123457.

First of all, it is important to note that this requirement is not suitable to be implemented with regularities, at least, regularities are not the best way to implement them.

This problem itself is not the focus of this paper, and what this paper is going to discuss is another problem of regular matching principle, which is mainly caused by the solution to this problem.

First take a look at the solution to the problem itself.

The code is as follows:

String str = "8912341253789"

Regex reg = new Regex (@ "((\ d)\ dink?)\ 2")

While (str! = (str = reg.Replace (str, "$1")) {}

RichTextBox2.Text = str

/ *-output-

89123457

, /

Based on this, another question raised by a friend is why the use of the following rules has no effect.

"(?