正则表达式不允许 a 和 c 彼此相邻

Question

I'm trying to write a regular expression which doesn't allow 'a' and 'c' to be next to each other in any combination of "abc", the combinations might be "a", "b", "c", "acb", "abac", here "abac" must be ignored because it contains "a" and "c" next to each other, I've written a regular expression which is doing half the job correct and the other half incorrect, it's basically ignoring a, bcb, bcc and others which are not supposed to be ignored.

Here's the regular expression:

^(a?b)*c?$

Here's the output I'm getting:

[a, b, c, ba, ca, ab, cb, ac, bc, baa, caa, aba, cba, aca, bca, 
bab, cab, abb, cbb, acb, bcb, bac, cac, abc, cbc, acc, bcc]
b 
c 
ab 
bc 
bab 
abb 
abc 

Could someone please tell me what I'm doing wrong?

Answer 1

Your expression ignores several cases:

• anything with more than one c is ignored
• anything with a or b coming after c is ignored (that means if there is a c that has to be the last character)
• anything containing an a is ignored if it doesn't contain also a b after that a
• each a must be followed by a b
• also your grouping is probably not really the form you need. You should use (?:X) for a non capturing group.

I would suggest a regex like

^(?:(?:a(?!c))?b?c?)+$

This matches all a s not followed by a c and also all b s and c s - and needs at least one occurence so that empty strings are not matched.

You can play with it and get detailed explanations at https://regex101.com/r/DoyUPG/1

Answer 2

Here is a much simpler straightforward regex. Rather than thinking to exclude the pattern, you can also match the pattern and ignore them like following example:

String[] str = {
    "a", "b", "c", "ba", "ca", "ab", "cb", "ac", "bc", "baa",
    "caa", "aba", "cba", "aca", "bca", "bab", "cab", "abb",
    "cbb", "acb", "bcb", "bac", "cac", "abc", "cbc", "acc", "bcc"
};
        
for(int i=0; i<str.length; ++i) {
    if(str[i].matches("ac.?|.?ac|ca.?|.?ca")) {
        System.out.println("MATCH: " + str[i]);
    } else {
        System.out.println(str[i]);
    }
}

This makes the following output:

a
b
c
ba
MATCH: ca
ab
cb
MATCH: ac
bc
baa
MATCH: caa
aba
cba
MATCH: aca
MATCH: bca
bab
MATCH: cab
abb
cbb
MATCH: acb
bcb
MATCH: bac
MATCH: cac
abc
cbc
MATCH: acc
bcc

Answer 3

In spite of your provided data, you said "the combinations might be "a", "b", "c", "acb", "abac", here "abac" which indicates they could be more than just three letters. Rather than use a regex I recommend String.contains .

String[] str = { "a", "b", "c", "babbacb", "ca", "ab", "cb",
        "aeseac", "bc", "baa", "caa", "aba", "cba", "aca",
        "bca", "bab", "cab", "abb", "cbbabcda", "acb",
        "bcbacbae", "bacadbac", "adecdcac", "abc", "cbc",
        "acc", "adbbcc", "abac" };
        
for (String s : str) {
    if (!(s.contains("ac") | s.contains("ca"))) {
        System.out.println(s);
    }
}

prints

a
b
c
ab
cb
bc
baa
aba
cba
bab
abb
cbbabcda
abc
cbc
adbbcc

But if you want to use a regex then simply check for those strings that matches string that is composed of at least one ac or ca .

String regex = ".*((ac)|(ca)).*";
for (String s : str) {
    if (!s.matches(regex)) {
        System.out.println(s);
    }
}

prints the same as above.