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  • Why is | called "infix operator", when it means "OR" here? Also, your answer will not output following lines, if they start with whitespace. Commented Jul 14, 2016 at 17:24
  • That is a good question for which I do not know the answer. I believe the pertinent information is contained in the second half of that sentence: the resulting regular expression matches any string matching either subexpression. As for a leading whitespace, it works on any line containing ERROR, FAIL, or WARN on my system. I have edited the answer to include GNU. Commented Jul 14, 2016 at 17:38
  • Ok, thanks for looking that up. I often use the word "infix", in the context "Reverse incremental infix search" (CTRL-R), but i had never seen it written in the context you quoted. 2) I meant lines, that start with whitespace, but follow a line containing the keyword, while not themselves containing the keyword. Commented Jul 14, 2016 at 17:44
  • This is the correct answer. +1. (I had misunderstood OP's desired input format.) Commented Jul 14, 2016 at 18:25
  • @AlexStragies I've sort of found the issue that has me saying some things aren't working. The actual log file that I want to pull errors from won't work with any of the commands in this thread, but the same exact text (copied/pasted word for word from the original) pasted into a fresh text file works. I have both read/write permissions, so I'm not sure why the original log or even duplicates of it won't obey. Commented Jul 14, 2016 at 22:24