trouble with writing regex java -

string consists of 2 distinct alternating characters. example, if string 's 2 distinct characters x , y, t xyxyx or yxyxy not xxyy or xyyx.

but a.matches() returns false , output becomes 0. me understand what's wrong here.

public static int check(string a) {     char on = a.charat(0);     char = a.charat(1);     if(on != to) {         if(a.matches("["+on+"("+to+""+on+")*]|["+to+"("+on+""+to+")*]"))  {              return a.length();         }                 }     return 0; } 

use regex (.)(.)(?:\1\2)*\1?.

• (.) match character, , capture group 1
• (.) match character, , capture group 2
• \1 match same characters captured in group 1
• \2 match same characters captured in group 2
• (?:\1\2)* match 0 or more pairs of group 1+2
• \1? optionally match dangling group 1

input must @ least 2 characters long. empty string , one-character string not match.

as java code, be:

if (a.matches("(.)(.)(?:\\1\\2)*\\1?")) { 

see regex101.com working examples¹.

^{1) note regex101 requires use of ^ , $, implied matches() method. requires use of flags g , m showcase multiple examples @ same time.}

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update

as pointed out austin anderson:

fails on yyyyyyyyy or xxxxxx

to prevent that, can add zero-width negative lookahead, ensure input doesn't start 2 of same character:

(?!(.)\1)(.)(.)(?:\2\3)*\2?

see regex101.com.

or can use austin anderson's simpler version:

(.)(?!\1)(.)(?:\1\2)*\1?