javascript - RegExp: Different results when using Backreference -

see http://jsfiddle.net/aeeun/

why when use backreference vs no backrefences, different results?

var str = "hello world\nhello          "; document.write("without backreference: <br />"); var match = str.match(/\s(?![\s\s]*\s)/); document.write("- match.index: " + match.index + "<br />"); // index 16   document.write("with backreference: <br />"); var match = str.match(/(\s)(?![\s\s]*\1)/); document.write("- match.index: " + match.index); // index 6 

the difference constrain 2 character same.

in first regular expression there must not any non-whitespace character following:

/\s(?![\s\s]*\s)/ 

this means match non-whitespace character not followed other non-whitespace character. that’s why matches last o followed whitespace:

"hello world\nhello          "                   ^ no other non-whitespace character following 

but in second regular expression there must not specific character following matched before:

/(\s)(?![\s\s]*\1)/ 

this means match non-whitespace character not appear again in rest of string. that’s why w matched it’s first non-whitespace character not appear again after first occurrence:

"hello world\nhello          "        ^ no other “w” following 

that’s why it’s called backreference: reference matched string.