Question

Why these return null :

var str="Is this all abc bbb c is";
var patt1=/is(?=bbb)/;

var str="Is this all abc bbb c is";
var patt1=/is(?=bbb)/;

var str="Is this all there is";
var patt1=/is(?=all)/; // <------ (?=all) vs (? =all)

but this returns is :

var str="Is this all there is";
var patt1=/is(?= all)/;

?

Answer 1

(?=...)

is the regex “look-ahead” function, so here is what your regular expressions mean:

/is(?=bbb)/ //look for isbbb and return is
/is(?=all)/ //look for isall and return is

Both of those do not exist in your string, so you get no matches.