问题
I have the following REGEX expression (that works) to allow Alpha-Numeric (as well as \' and -) and no double spacing:
^([a-zA-Z0-9\'-]+\\s?)*$
Due to the nested grouping, this allows Catastrophic Backtracking to happen - which is bad!
How can I simplify this expression to avoid Catastrophic Backtracking?? (Ideally this wouldn\'t allow white-space in first and last characters either)
回答1:
Explanation
Nested group doesn't automatically causes catastrophic backtracking. In your case, it is because your regex degenerates to the classical example of catastrophic backtracking (a*)*.
Since \s in optional in ^([a-zA-Z0-9'-]+\s?)*$, on input without any spaces but has characters outside the allowed list, the regex simply degenerates to ^([a-zA-Z0-9'-]+)*$.
You can also think in term of expansion of the original regex:
[a-zA-Z0-9'-]+\s?[a-zA-Z0-9'-]+\s?[a-zA-Z0-9'-]+\s?[a-zA-Z0-9'-]+\s?...
Since \s is optional, we can remove it:
[a-zA-Z0-9'-]+[a-zA-Z0-9'-]+[a-zA-Z0-9'-]+[a-zA-Z0-9'-]+...
And we got a series of consecutive [a-zA-Z0-9'-]+, which will try all ways to distribute the characters between themselves and blow up the complexity.
Solution
The standard way to write a regex to match token delimiter token ... delimiter token is token (delimiter token)*. While it is possible to rewrite the regex avoid repeating token, I'd recommend against it, since it is harder to get it right. To avoid repetition , you might want to construct the regex by string concatenation instead.
Following the recipe above:
^[a-zA-Z0-9'-]+(\s[a-zA-Z0-9'-]+)*$
Although you can see repetition in repetition here, there is no catastrophic backtracking, since the regex can only expand to:
[a-zA-Z0-9'-]+\s[a-zA-Z0-9'-]+\s[a-zA-Z0-9'-]+\s[a-zA-Z0-9'-]+...
And \s and [a-zA-Z0-9'-] are mutual exclusive - there is only one way to match any string.
来源：https://stackoverflow.com/questions/27237579/simple-alphanumeric-regex-single-spacing-without-catastrophic-backtracking