Why does splitting on a period need double back-slashes?

Question

I have a String called filename:

filename = "z_cams_c_ecmf_20170217000000_prod_fc_pl_015_aermr04.nc";

When I try to split the filename to get the variable name aermr04.nc, I tried the following:

String varibleName = filename.split("_")[9].split(".")[0];

The above line of code throws an IndexOutOfBoundsException.

Why?

I can get it tow work by using:

String varibleName = filename.split("_")[9].split("\\.")[0];

However, it seems rather silly that I have to fiddle around with such trivial tasks...

Any idea why the 2nd example works? What is the reasoning behind such syntax?

Because the period `.`, in a regex, means "any character". So for a literal period, it needs to be escaped (i.e. preceded by a backslash). But since the backslash '\' in a string needs to be escaped, it becomes `"\\."`. — AntonH, Feb 17 '17 at 20:46
Note that you can do this without regex or creating all the unnecessary strings and arrays, using `String varibleName = filename.substring(filename.lastIndexOf('_'), filename.lastIndexOf('.'))` (assuming those really are the last `_` and `.` in your string). — Andy Turner, Feb 17 '17 at 20:53

score 1 · Accepted Answer · edited Feb 17 '17 at 20:50

1

The argument to .split() is treated as a regular expression. "." as a regex matches everything.

To match a period, you need to escape the "." regex as "\\."

edited Feb 17 '17 at 20:50

Andy Turner

answered Feb 17 '17 at 20:47

Bryan Karlovitz

1

Note that using code formatting shows up your double backslashes. – Andy Turner Feb 17 '17 at 20:50

1 Answers1