Question

I am trying to read someone else’s code and I came across this piece of regex. Can anyone tell whats the author trying to do ?

if(str.matches("^\\s*$") && !str.matches("^\\s*#.*")) {
       // do something
}

Answer 1

The first pattern looks for an empty line – a line containing only white space (spaces, tabs, etc).

The second pattern looks for lines starting with white space followed by a # (hash), and ignores them.

It’s odd; anything that matches the first regex won’t match the second, so the second test is pointless. To make sense, the test should probably be:

if (!str.matches("^\\s*$") && !str.matches("^\\s*#.*")) {
       // do something with non-blank, non-comment lines
}

Or:

if (str.matches("^\\s*$") || str.matches("^\\s*#.*")) {
       // do something with all blank or all comment lines (like ignore them)
}