正则表达式，带有所有可选部件，但至少需要一个 [英] Regex with all optional parts but at least one required

问题描述

我需要编写一个正则表达式来匹配诸如 abc， ab， ac， bc， a， b， c之类的字符串。顺序很重要，并且不应该与同一部分的多个外观匹配。

I need to write a regex that matches strings like "abc", "ab", "ac", "bc", "a", "b", "c". Order is important and it shouldn't match multiple appearances of the same part.

a？b？c？几乎可以解决问题。除了它也匹配空字符串。有什么方法可以防止它匹配空字符串，或者有其他方法可以为任务编写正则表达式。

a?b?c? almost does the trick. Except it matches empty strings too. Is there any way to prevent it from matching empty strings or maybe a different way to write a regex for the task.

推荐答案

到使用纯正则表达式来做到这一点，您将不得不将其扩展到所有可能性：

To do this with pure regex you're going to have to expand it into all of its possibilities:

ab?c?|a?bc?|a?b?c

如果有前瞻，则可以确保字符串不是-空。或者，您可以在将字符串传递给表达式之前验证其长度至少为一，具体取决于您选择的语言。

If you have lookaheads you can make sure the string is non-empty. Or you can verify the string has a length of at least one before passing it to the expression, depending on your choice of language.

例如，.NET lookahead可能看起来像像这样：

For example a .NET lookahead might look like this:

^(?=[abc])a?b?c?$

或者您可以在匹配字符串之前测试一下字符串的长度：

Or you could just test your string's length before matching it:

if (YourString.Length == 1) {
   // matching code goes here, using the expression a?b?c?
}

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