str = '[2019-07-29 09:48:11,928] @hr.com [2] [AM] WARN'
result = ''
cnt = 0
str.scan(/\[([^\]\[]*)\]/) { |match| result = match[0]; cnt +=1; break if cnt >= 2}
puts result #=> 2
c3RyID0gJ1syMDE5LTA3LTI5IDA5OjQ4OjExLDkyOF0gQGhyLmNvbSBbMl0gW0FNXSBXQVJOJwpyZXN1bHQgPSAnJwpjbnQgPSAwCnN0ci5zY2FuKC9cWyhbXlxdXFtdKilcXS8pIHsgfG1hdGNofCByZXN1bHQgPSBtYXRjaFswXTsgY250ICs9MTsgYnJlYWsgaWYgY250ID49IDJ9CnB1dHMgcmVzdWx0ICM9PiAyCg==