regex - grep with regexp: whitespace doesn't match unless I add an assertion

Question

GNU grep 2.5.4 on bash 4.1.5(1) on Ubuntu 10.04

This matches

$ echo "this is a     line" | grep 'a[[:space:]]+line'
this is a     line

But this doesn't

$ echo "this is a     line" | grep 'as+line'

But this matches too

$ echo "this is a     line" | grep 'as+line'
this is a     line

I don't understand why #2 does not match (whereas # 1 does) and #3 also shows a match. Whats the difference here?

Answer 1

grep doesn't support the complete set of regular expressions, so try using -P to enable perl regular expressions. You don't need to escape the + i.e.

echo "this is a     line" | grep -P 'as+line'