Java's Regex Grouping

I recently used Java’s regex (regular expression) matching and powerful grouping feature and yet again remembered just how powerful it can be.
I figured I’d do a recap here for prosperity.

Creating a Pattern and a Matcher

In this example I’m going to match on two simple numeric strings with a limit of 5 characters each and separated by a hyphen, e.g. 12345-67890.

First you create a regex to match the behaviour we want
Then compile the regex into a Pattern object
This pattern object can then be used on an input string to create a Matcher
```
String myNumeric = "12345-67890";
Pattern myPattern = Pattern.compile("[0-9]{5}-[0-9]{5}");
Matcher myMatcher = myPattern.matcher(myNumeric);
myMatcher.matches();
```
A Matcher has a some key operations:
- matches() used to confirm a match for the entire input string, returning a boolean
- find() to find the next match to our pattern within the next subsequence of the input string.

A Pattern can be reused for multiple input strings, the matcher however is particular to the input string.

Grouping

To create a regex that supports grouping you need to make use of parentheses () (also known as brackets) to bound your group.
Each set of parentheses will mark a group.
Groups are identified by a number and are counted from the left most opening parantheses to the right most.
In order to access the groups via myMatcher.group(int group); you will need to either call matches() or find() to parse the string with our regex.

Below are two examples of grouping using the same general pattern.

([0-9]{5})-([0-9]{5})
Group 0 = whole content of regex, group 1 = first group of five numbers, group 2 = second group of five numbers Printing out the groups (0,1,2) produces:

12345-67890
12345
67890

(([0-9]{5})-)([0-9]{5})
Group 0 = whole content of regex, group 1 = first group of five numbers incl the hyphen, group 2 = first group of five numbers, group 3 = second group of five numbers Printing out the groups (0,1,2,3) produces:

12345-67890
12345-
12345
67890

Grouping by name

Grouping by name still makes use of the parentheses but includes additional syntax for assigning names.
Using a question mark ? followed by a name within two square brackets (<>) will denote your grouping name.

(?<first>[0-9]{5})-(?<second>[0-9]{5}) Here we’ve added two names; first and second to our existing groups.
Now we can use matcher.group("first") and matcher.group("second") to view these groups, producing the following output:

12345 67890

Closing thoughts

Using grouping can make debugging and building up a regex much easier, you can print off the group content to see what it matches on and then continue with your expression.
Using the naming feature makes following the matching much easier for other people to read your regex and understand your intentions.