Problems using + with -f <file>

[Gert Jan van Loo]

My source contains many of:
Input rstn is unused       
Input location is unused   
Input rstn is unused       
Input n is unused          

My pattern file contains (among other lines):
Input [a-zA-Z0-9_]+ is unused
 
If I use "grep -f <patternfile> <source>" The patterns are NOT found 
If I use "egrep -f <patternfile> <source>" The patterns ARE found
 
If I change my pattern file to:
Input [a-zA-Z0-9_]\+ is unused
Now using "grep -f <patternfile> <source>" The patterns ARE found.
Now using "egrep -f <patternfile> <source>" The patterns are NOT found.

egrep seems to do the right thing. 
I thought "[a-zA-Z0-9_]+" is part of the standard grep.
Even if it is not why does \+ suddenly work? 

Version:
grep (GNU grep) 2.5.1
on Linux Redhat.

  
- G.J. van Loo
- Principle ASIC Engineer Broadcom Ltd.