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 ?

CodePudding user response：

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

CodePudding user response：

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 as not followed by a c and also all bs and cs - 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

CodePudding user response：

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.