You cannot skip a character in a capture group. A match is always consecutive, even when it contains things like zero-width assertions.
However, you can access the matched groups in a regular expression like the following code −
<html> <head> <script> var str = "Username akdg_amit"; var myReg = /(?:^|\s)akdg_(.*?)(?:\s|$)/g; var res = myReg.exec(str); document.write(res); </script> </head> <body> </body> </html>