Java Regex：仅允许某些字符，但不允许某些字符以字符串开头？

Question

I'm fiddling around with Java regex, and I'm trying to come up with a pattern that allows a certain set of characters anywhere else BUT it can't start with certain characters in the allowed set.

For example, let's say the allowed characters are from A to Z, but the string can't start with X or Z. How do I do that? I've come up with ^[XZ][^AZ]+ , and while it works otherwise, it allows the string to start with other letters that are not in the set (eg with punctuation).

Answer 1

You can use this regex:

^[A-WY][A-Z]*$

• ^[A-WY] ensures that the first character is AW or Y
• [AZ]*$ will match 0 or more of any uppercase English letter

In general to exclude certain characters you can also use negative look-ahead:

^(?![XZ])[A-Z]+$

(?![XZ]) is negative lookahead to disallow X or Z at start.

Answer 2

Java regexes support subtraction in character classes; see http://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html , which shows these as examples:

[a-z&&[^bc]]    a through z, except for b and c: [ad-z] (subtraction)
[a-z&&[^m-p]]   a through z, and not m through p: [a-lq-z](subtraction)

So you could say

[A-Z&&[^XZ]]

to mean any upper-case ASCII letter except X or Z. It really isn't needed here, but if you're using large classes like Posix character classes, it could be more useful.

Warning: Not all languages support this construct in regexes. I'm pretty sure C++ and Javascript don't, and I don't actually know of another language that does but I haven't checked.