regex to return all values not just first found one - regex

Question

I'm learning Pig Latin and am using regular expressions. Not sure if the regex is language agnostic or not but here is what I'm trying to do.
If I have a table with two fields: tweet id and tweet, I'd like to go through each tweet and pull out all mentions up to 3.
So if a tweet goes something like "#tim bla #sam #joe something bla bla" then the line item for that tweet will have tweet id, tim, sam, joe.
The raw data has twitter ids not the actual handles so this regex seems to return a mention (.*)#user_(\\S{8})([:| ])(.*)
Here is what I have tried:
a = load 'data.txt' AS (id:chararray, tweet:chararray);
b = foreach a generate id, LOWER(tweet) as tweet;
// filter data so only tweets with mentions
c = FILTER b BY tweet MATCHES '(.*)#user_(\\S{8})([:| ])(.*)';
// try to pull out the mentions.
d = foreach c generate id,
REGEX_EXTRACT(tweet, '((.*)#user_(\\S{8})([:| ])(.*)){1}',3) as mention1,
REGEX_EXTRACT(tweet, '((.*)#user_(\\S{8})([:| ])(.*)){1,2}',3) as mention2,
REGEX_EXTRACT(tweet, '((.*)#user_(\\S{8})([:| ])(.*)){2,3}',3) as mention3;
e = limit d 20;
dump e;
So in that try I was playing with quantifiers, trying to return the first, second and 3rd instance of a match in a tweet {1}, {1,2}, {2,3}.
That did not work, mention 1-3 are just empty.
So I tried changing d:
d = foreach c generate id,
REGEX_EXTRACT(tweet, '(.*)#user_(\\S{8})([:| ])(.*)',2) as mention1,
REGEX_EXTRACT(tweet, '(.*)#user_(\\S{8})([:| ])(.*)#user_(\\S{8})([:| ])(.*)',5) as mention2,
REGEX_EXTRACT(tweet, '(.*)#user_(\\S{8})([:| ])(.*)#user_(\\S{8})([:| ])(.*)#user_(\\S{8})([:| ])(.*)',8) as mention3,
But, instead of returning each user mentioned, this returned the same mention 3 times. I had expected that by cutting n pasting the expression again I'd get the second match, and pasting it a 3rd time would get the 3rd match.
I'm not sure how well I've managed to word this question but to put it another way, imagine that the function regex_extract() returned an array of matched terms. I would like to get mention[0], mention[1], mention[2] on a single line item.

Answer 1

Whenever you use PATTERN_EXTRACT or PATTERN_EXTRACT_ALL udf, keep in mind that it is just pure regex handled by Java.
It is easier to test the regex through a local Java test. Here is the regex I found to be acceptable :
Pattern p = Pattern.compile("#(\\S+).*?(?:#(\\S+)(?:.*?#(\\S+))?)?");
String input = "So if a tweet goes something like #tim bla #sam #joe #bill something bla bla";
Matcher m = p.matcher(input);
if(m.find()){
for(int i=0; i<=m.groupCount(); i++){
System.out.println(i + " -> " + m.group(i));
}
}
With this regex, if there is at least a mention, it will returns three fields, the seconds and/or third being null if a second/third mention is not found.
Therefore, you may use the following PIG code :
d = foreach c generate id, REGEX_EXTRACT_ALL(
tweet, '#(\\S+).*?(?:#(\\S+)(?:.*?#(\\S+))?)?');
You do not even need to filter the data first.