Java + Regex：匹配自定义集合中的字符，这些字符前面没有同一集合中的字符

Question

I am running into a silly problem with Regular Expressions in Java where I would like to match a String that begins with @ with characters from a certain valid set, but are not preceded by characters from the same valid set.

The terms I would like to match are of the form:

"y" + @ + "xxxxxxx"

where:

• x is a character that belongs to the valid set [a-zA-Z\\\\d\\\\-\\\\_]
• the @ sign appears once
• y is a character that does not belong to the valid set [a-zA-Z\\\\d\\\\-\\\\_]

I'm currently trying to do this this by using the following regular expressions pattern

MY_PATTERN = "[^[A-Za-z\\d\\-\\_]?]" + "@{1}" + "[A-Za-z\\d\\-\\_]+"
String text = "12a@cat123-_     @dog123__- ";
Pattern p = Pattern.compile(PATTERN);
Matcher m = p.matcher(text);

Based on this I expect the following code to only print @dog123__-

while(m.find()){ String s = m.group(); System.out.println(s); }

However it also prints out a@cat123-_ .

Could someone explain what I'm doing wrong?

Answer 1

I'm assuming the text you're trying to match could be anywhere, and not anchored to the start of the string.

The syntax you used for [^[A-Za-z\\\\d\\\\-\\\\_]?] is wrong and is being interpreted as something else (let's not get into that). Negated character classes are [^chars] . So the syntax should have been [^A-Za-z\\\\d\\\\-_] . However, that requires to match that character before the "@" , so it won't match "@foo" , because "there isn't a character (that is not A-Za-z0-9-_) before".

Lookbehinds to the rescue. A negative lookbehind (?<!subpattern) specifies the current position is not preceded by subpattern.

Oh, and one more thing, [A-Za-z\\\\d\\\\-_] is the same as [-\\\\w] (let's use that shorter version).

So the regex should be:

(?<![-\\w])@[-\\w]+

ideone Demo

Answer 2

You have some issues in your pattern, here's one that should do it:

(?:^|[^A-Za-z\d\-\_])(@[A-Za-z\d\-\_]+)

1. @{1} it's the same as @
2. [^[A-Za-z\\d\\-\\_]?] The problem seems to be here, you're using nested character sets, which doesn't work
3. It should be [^A-Za-z\\d\\-\\_]

You could simplify the regex to: (?:^|[^\\w\\-])(@[\\w\\-]+)

\\w matches any alphanumeric character & underscore

Test this: http://regexr.com/3bt77

It's javascript, but you shouldn't have any issues.

Answer 3

Your regex can be simplified considerably, given:

[a-zA-Z\\d\\-\\_] === [\w-]

so this is what what you want:

[^\w-]@[\w-]