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

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 have got a space after them. I used following expression and it worked

grep -o  [[:alnum:]]*.[[:blank:]]  test.txt

Its output is

odsdsdoddf112
dad23392eeedJ 
test2 
for    
Hello 
World

But problem is grep prints all the strings that have got space after them, where as I want it to stop after first match on a line and then proceed to second line.

Which expression should I use here, in order to make it stop after first match and move to next line?

This problem may be solved with gawk or some other tool, but I will appreciate a solution which uses grep only.

Edit I using GNU grep 2.5.1 on a Linux system, if that is relevant.

Edit

With the help of the answers given below, I tried my luck with

grep  -o   ^[[:alnum:]]*  test.txt
grep  -Eo  ^[[:alnum:]]+  test.txt

and both gave me correct answers.

Now what surprises me is that I tried using

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

as suggested here but didn't get the correct answer. Here is the output on my terminal

odsdsdoddf112
dad23392eeedJ 
test2 
for    
Hello 
World

But comments from RichieHindle and Adrian Pronk, shows that they got correct output on their systems. Anyone with some idea that why I too am not getting the same result on my system. Any idea? Any help will be appreciated.

Edit

Well, it seems that grep 2.5.1 has some bug because of which my output wasn't correct. I installed grep 2.5.4, now it is working correctly. Please see grep