[laimas]

var s = ' abcde abc5 abc0 abcfg'.trim(), 
    w = s.split(/\s+/); 

for(var i=1; i < w[0].length; ++i) if(s.match(new RegExp('\\b'+w[0].substr(0, i), 'g')).length < w.length) break;

console.log(i)