How to make grep stop at first match on a line?

Question

Well, I have a file test.txt

#test.txt
odsdsdoddf112 test1_for_grep
dad23392eeedJ test2 for grep
Hello World test
garbage

I want to extract strings which h

Answer 1

If we want to extract all meaningful input before garbage and actually stop on first match then -B NUM, --before-context=NUM option may be useful to "print NUM lines of leading context before matching lines".

Example:

grep --before-context=999999 "Hello World test"

Answer 2

grep -oe "^[^ ]* " test.txt

Answer 3

If you're sure you have no leading whitespace, add a ^ to match only at the start of a line, and change the * to a + to match only when you have one or more alphanumeric characters. (That means adding -E to use extended regular expressions).

grep -Eo "^[[:alnum:]]+[[:blank:]]" test.txt

(I also removed the . from the middle; I'm not sure what that was doing there?)

Answer 4

As the questioner discovered, this is a bug in versions of GNU grep prior to 2.5.3. The bug allows a caret to match after the end of a previous match, not just at beginning of line.

This bug is still present in other versions of grep, for instance in Mac OS X 10.9.4.

There isn't a universal workaround, but in the some examples, like non-spaces followed by a space, you can often get the desired behavior by leaving off the delimiter. That is, search for '[^ ]*' rather than '[^ ]* '.