grep doesn't work as expected

Question

I'm trying to understand how grep deciding to take $ as regex, or as a searchable character. My results show that grep is inconsistently deciding what $ means.

As such, I made a basic example:

$ cat testfile
$illy
$unset

Here I want to see if I can grab the line ending in "illy" using regex:

$ grep 'illy$' testfile
$illy

It seems to work, and it is interpreting the 'illy$' criteria as the $ being a regex to say "match this expression at the end of the line". So my thinking now, is that grep interprets $ as a regex criteria, and not a string. So if I wanted to try and grep for '$illy', then this should not match anything because it's interpreting $ as regex.

However:

$ grep '$illy' testfile
$illy

Why does it find this line? This proves that even grep doesn't know how to interpret the character $. Let alone, myself trying to understand it.

Answer 1

grep by default uses Basic Regular Expressions (BRE), and $ is a special character in BRE's only at the end of an expression.

If you want grep to process pattern as an extended regexp, use -E option

-E, --extended-regexp
              Interpret PATTERN as an extended regular  expression  (ERE,  see
              below).

-

pilot6@Pilot6:~$ grep '$illy' test
$illy
pilot6@Pilot6:~$ grep -E '$illy' test
pilot6@Pilot6:~$

pilot6@Pilot6:~$ grep 'illy$' test
$illy
pilot6@Pilot6:~$ grep -E 'illy$' test
$illy