Regex Matching, when there is a pattern moving backward in the target String?

Question

I have a file with each line having the following pattern:

A<space>B<space>C

where:

• A,B,C are three blocks of variable length Strings, for example "Hi I'm block A I'mB I'mC"
• B and C are each one word, C ends with line termination and there are "spaces" around B.
• A can pretty much be anything (can also have any variable number of spaces)

What regex pattern can I use to split the line in 3?!

I currently use something like: substring(*line*.lastIndexOf(" ")) to get C, changing line to hold A B and repeat to get B, and then whatever is left is A.

But is there a way to do it with Regex?! In general how can Regex be used when the pattern is know moving backward in a string?!

Answer 1

This regex will do it:

^(.+?) (\S+) (\S+)$

Here's a demo. In your example, it captures three groups:

Hi I'm block A
I'mB
I'mC

Explanation:

• ^ start of the string
• (.+?) capture any characters, but "non-greedy" (stop as soon as possible)
• a space
• (\S+) capture any group of non-whitespace characters
• a space
• (\S+) again, capture any group of non-whitespace characters
• $ end of the line

Answer 2

Just do splitting on the first and second space from last.

string.split("\\s+(?=\\S+\\s+\\S+$)|\\s+(?=\\S+$)");

DEMO