Regex for overlapping fixed width matches -

so have string:

'aaggbb'

and want find groups of type axxxb x char. thought regex:

/(a(?:...)b)/ig

would trick, gets first one:

'aaggbb'

and misses second one:

'aaggbb'

how both?

i searched around while trying figure out hope didn't miss obvious. thanks!

if looking fixed length substrings, need specifify limiting quantifier {3} (match 3 symbols) , use capturing inside look-ahead match substrings:

(?=(a.{3}b)) 

see demo

the look-ahead not consuming characters, , enable overlapping matches. "consuming" means after lookahead or lookbehind's closing parenthesis, regex engine left standing on same spot in string started looking: hasn't moved. position, engine can start matching characters again. (from rexegg.com)

var re = /(?=(a.{3}b))/g;   var str = 'aaggbb';    while ((m = re.exec(str)) !== null) {      if (m.index === re.lastindex) {          re.lastindex++;      }      document.getelementbyid("demo").innerhtml += m[1] + "<br/>";  }

<div id="demo" />