Note that you can use " " to enclose the pattern or ' '.
Use of ' ' allows you to substitute for shell variables.
In general " " is weaker than ' '. 

grep -n '.*' reg.txt
grep -n '^.*' reg.txt
	1:However, this is too late.
	2:It may well be the end.
	3:My area code is (626)
	4:213 was the old area code for LA.
	5:
	6:This is an htlm fragment: <a href="HelloKitty.txt"> Hello.txt</a>
	7:             
	8:

grep -n '^However' reg.txt
	1:However, this is too late.

grep -n 'end.$' reg.txt
	2:It may well be the end.

grep -n '[0-9]' reg.txt        #Search for one digit
	3:My area code is (626)
	4:213 was the old area code for LA.

grep -n '[0-9][0-9]*' reg.txt  #Search for one or more digits
	3:My area code is (626)
	4:213 was the old area code for LA.

grep -n '^$' reg.txt	#blank line
	5:
	8:

grep -n '^  *$' reg.txt	#white space line
	7:   

grep -n "[0-9]\{3\}-[0-9]\{2\}-[0-9]\{4\}"  #Social Security Number

grep -n '<[^>].*>' reg.txt                  #Simple HTML tag (same line)

grep -n '<[^>].*/a>' reg.txt
grep -n '<[^>].*\/a>' reg.txt
grep -n '<[^>].*\a>' reg.txt
	6:This is an htlm fragment: <a href="HelloKitty.txt"> Hello.txt</a>