REGEX in java for extracting consecutive duplicate characters in a string

Question

Question: Print character(s) in a string that are repeated consecutively only twice (not more).

Examples:

1)"aaabbaa" : b and a
2)"aabbaa" : a and b and a
3)"abba" : b

Code I tried:

String str = "aabbbbcccd";
Pattern p = Pattern.compile("(\w){2}");
Matcher m = p.matcher(str);
while(m.find())
{
System.out.println(m.group(1));
}

Output:
a
b
b
c
d
Although, the desired output is
a
d

Postscript
As I have recently started with regex, it would highly appreciated if the answerer can explain
the regex used briefly (especially quantifiers and groups).

Wiktor Stribiżew · Accepted Answer · 2021-07-23 10:54:05Z

3

There is no single plain regex solution to this problem because you need a lookbehind with a backreference inside, which is not supported by Java regex engine.

What you can do is either get all (\w)\1+ matches and then check their length using common string methods:

String s = "aaabbaa";
Pattern pattern = Pattern.compile("(\\w)\\1+");
Matcher matcher = pattern.matcher(s);
while (matcher.find()){
    if (matcher.group().length() == 2) System.out.println(matcher.group(1)); 
}

(see the Java demo) or you can match 3 or more repetitions or just 2 repetitions and only grab the match if the Group 2 matched:

String s = "aaabbaa";
Pattern pattern = Pattern.compile("(\\w)\\1{2,}|(\\w)\\2");
Matcher matcher = pattern.matcher(s);
while (matcher.find()){
    if (matcher.group(2) != null)
        System.out.println(matcher.group(2)); 
}

See this Java demo. Regex details:

(\w)\1{2,} - a word char and two or more occurrences of the same char right after
| - or
(\w)\2 - a word char and the same char right after.

answered Jul 23, 2021 at 10:54

Wiktor Stribiżew

631k41 gold badges502 silver badges632 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

believer Over a year ago

Of course. I had some doubts: 1) In the first case, instead of using <<(\\w)\\1+>> can I use <<"(\\w){1,}">> or <<"(\\w){2}">> 2) Does <<\\1>> tells the compiler that it is for group 1. 3) Can I use <<{1,}>> instead of <<+>>

Wiktor Stribiżew Over a year ago

@believer {1,} is the same as +, it repeats the pattern it modifies one or more times. \w+ matches one or more letters/digits/_, and thus matches ab123_any___, 123, _, etc. The \1 in (\w)\1 is a backreference to the Group 1 value, and only matches what has been captured. Use whatever you need, it only depends on your real requirements, but one thing is certain: you cannot just change \1 with +.

Collectives™ on Stack Overflow

REGEX in java for extracting consecutive duplicate characters in a string

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related