strange regex matching with grep/egrep

Question

• GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
• grep (GNU grep) 2.20
• grep-2.20-3.el7.x86_64

Can someone explain this puzzle? I'm getting false matches with grep/egrep.

echo "somestringthing" | egrep  '\bstring*'
(no output as expected)
echo "somestringthing" | egrep '\bsomestring*'
somestringthing
echo "somestringthing" | egrep '\bsomestringthingy*'
somestringthing
echo "somestringthing" | egrep '\bsomestringthing1*'
somestringthing
echo "somestringthing" | egrep '\bsomestringthingX*'
somestringthing

That last three should NOT match because of the single char before the wildcard. Experimenting, I've found that any string will match as if the single character before the wildcard did not exist.

'\b' is a word boundary, FYI.

So am I missing something here, or is this a bug in grep? (Talk about hair-pulling madness trying to debug code you think is working properly.)

Answer 1

The y*, 1* and X* at the end of the last three regular expressions will match zero or more y, 1 and X respectively.

At the end of the input string somestringthing you do actually have zero or more of these characters (exactly zero), so all three expressions matches.

If you want to match one or more y at the end of the string, use y+ or y{1,} in an extended regular expression, or yy* or y\{1,\} in a basic regular expression (grep without -E):

echo somestringthing | grep -E 'somestringthingy+'

(this produces no output)

Also note that egrep is deprecated and you should be using grep -E. If you want to match complete words only, use grep -E -w (this would require a word boundary at the start and end of the match in the input).

Answer 2

Bahh..more messing around and it seems the character before the * wildcard is being treated as a .

The proper wildcard use for grep is apparently .* not just *

Also, the \b was not required once I used the .* as the wildcard. The -w flag works as expected:

echo "somestringthing" | egrep -w 'somestring.*'
somestringthing

echo "somestringthing" | egrep -w 'somestringy.*'
(no output as expected)