Can someone please explain this JavaScript regular expression for me?

Question

Can someone please explain this JavaScript regular expression for me?

new RegExp("(^|\\s+)" + className + "(\\s+|$)"), ' ')

Answer 1

(             Either
  ^               the start of the string
|               or
  \\s+            one or more whitespace characters
)             followed by
className       the class name in question
(             followed by either
  \\s+          one or more whitespace characters
|             or
  $             the end of the string
)

So it will match "pog" in:

"pog"
"  pog"
"pog  "
"pog bim"
"bim pog"
"  pog bim"
"bim pog  "
"bim pog pam"

etc.

The second argument to new RegExp() can give options, eg. "i" meaning "case insensitive". In your case you're not passing any options (which is correct if you're dealing with HTML class names - class names should be treated case-sensitively).