var str="java";
var arr = str.split('');
var reg='';

for(var i=0; i<arr.length-1; i++){
if(i)reg+='|';
reg+=arr[i]+'(?='+arr[i+1]+')';
}
reg = new RegExp(reg,'g');
alert(reg)

var re = /ja|av|va/g;
var s = "java";
var allResults = [];

var result;

while( result = re.exec( s ) )
{
  allResults.push( result[0] );
  re.lastIndex = result.index + 1;
}
alert( allResults );