How abbrev pattern parameter works?

How abbrev pattern parameter works?

This code from https://github.com/rubinius/rubinius/blob/master/lib/abbrev.rb
def abbrev(words, pattern = nil)
table = {}
seen = Hash.new(0)
if pattern.is_a?(String)
pattern = /^#{Regexp.quote(pattern)}/ # regard as a prefix
end
words.each do |word|
next if (abbrev = word).empty?
while (len = abbrev.rindex(/[\w\W]\z/)) > 0
abbrev = word[0,len]
next if pattern && pattern !~ abbrev
case seen[abbrev] += 1
when 1
table[abbrev] = word
when 2
table.delete(abbrev)
else
break
end
end
end
words.each do |word|
next if pattern && pattern !~ word
table[word] = word
end
table
end
I'm not sure how pattern parameter is working, Can someone please explain
it to me with an example?