Regex that does not accept sub strings of more than two 'b'

user_17440991 0
Viewed 63

I need a regex that accepts all the strings consisting only of characters a and b, except those with more than two 'b' in a row.

For example, these should not match:

abb
ababbb
bba
bbbaa
bbb
bb

I came up with this, but it's not working

[a-b]+b{2,}[a-b]*

Here is my code:

int main() {
    string input;
    regex validator_regex("\b(?:b(?:a+b?)*|(?:a+b?)+)\b");
    
    cout << "Hello, "<<endl;
    while(regex_match(input,validator_regex)==false){
        cout << "please enter your choice of regEx :"<<endl;
        cin>>input;
        if(regex_match(input,validator_regex)==false)
            cout<<input+" is not a valid input"<<endl;
        else
            cout<<input+" is  valid "<<endl;
    }
}
regression regex
2 Answers

Your pattern [a-b]+b{2,}[a-b]* matches 1 or more a or b chars until you match bb which is what you don't want. Also note that the string should be at least 3 characters long due to this part [a-b]+b{2,}

To not match 2 b chars in a row you can exclude those matches using a negative lookahead by matching optional chars a or b until you encounter bb

Note that [a-b] is the same as [ab]

\b(?![ab]*?bb)[ab]+\b
  • \b A word boundary
  • (?![ab]*?bb) Negative lookahead, assert not 0+ times a or b followed by bb to the right
  • [ab]+ Match 1+ occurrences of a or b
  • \b A word boundary

Regex demo

Without using lookarounds, you can match the strings that you don't want by matching a string that contains bb, and capture in group 1 the strings that you want to keep:

\b[ab]*bb[ab]*\b|\b([ab]+)\b

Regex demo

Or use an alternation matching either starting with b and optional repetitions of 1+ a chars followed by an optional b, or match 1+ repetitions of starting with a followed by an optional b

\b(?:b(?:a+b?)*|(?:a+b?)+)\b

Regex demo

user_54249880

The simplest regex is:

^(?!.*bb)[ab]+$

See live demo.

This regex works by adding a negative look ahead (anchored to start) for bb appearing anywhere within input consisting of a or b.

If zero length input should match, change [ab]+ to [ab]*.

user_2561960
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