Java replaceAll() method

Question

I am trying to understand this piece of codes:

for (File f : files) {
 fileName = f.getName().toUpperCase().replaceAll("_\\d\\d\\d\\d_\\d\\d_\\d\\d_\\d\\d_\\d\\d", "");
 if (fileName.equals(tableName + ".XML")) {
 returnFile = f;
 break;
 }
}

and I am stuck at this part: replaceAll("_\\d\\d\\d\\d_\\d\\d_\\d\\d_\\d\\d_\\d\\d", "")

as far as I know it is trying to remove something from the name (maybe the underscore "_" ) but what exactly is _\\d\\d\\d\\d_\\d\\d_\\d\\d_\\d\\d_\\d\\d

Can somebody please explain?

• SLaks: Learn about regexes.
• user1531971: This code now has two problems.
• Ronaldinho Learn Coding: @jdv what are those two problems you are seeing?
• user1531971: blog.codinghorror.com/…

Answer 1

str.replaceAll("_\\d\\d\\d\\d_\\d\\d_\\d\\d_\\d\\d_\\d\\d", "") takes the string str and replaces all matches of the regular expression _\d\d\d\d_\d\d_\d\d_\d\d_\d\d with nothing (i.e. ""). (The reason it is written \\d and not \d is that the \ is escaped.)

In this case, \d means "a digit". So, more than likely, it removes a date/time from the string. For an example, if str is "screenshot_from_stackoverflow_2016_03_30_23_47.jpg", it becomes screenshot_from_stackoverflow.jpg after the replaceAll.

To get a feel for regular expressions, how they work and what they can do, I would recommend reading up on them, for instance on regular-expressions.info. It has a pretty comprehensive tutorial available.

• ninja.coder: Actually, it will become SCREENSHOT_FROM_STACKOVERFLOW.JPG as he is also doing toUpperCase() before replaceAll().
• Sebastian Paaske Tørholm: @user2004685 In the example provided in the question, yes. :) This answer only really covers the replaceAll-part, as it was the part being asked about.
• ninja.coder: I'm not correcting you. Just saying for common understand if anyone is interested. :)

Answer 2

as far as I know it is trying to remove something from the name (maybe the underscore "_" ) but what exactly is \d\d\d\d\d\d_\d\d_\d\d_\d\d

It is the pattern which will match the digits [0-9] in this format _XXXX_XX_XX_XX_ and replace it with "" i.e. nothing.

For Example,

_7686_77_78_77_77
_0123_65_58_56_12

Will be replaced in your string with "".

Answer 3

\d - matches a digit [0-9]

Java RegEx