7. Regular Expressions

A regular expression defines a set of one or more strings of characters. Many Unix utilities, including grep, ls, and find, use regular expressions to search for and replace strings. A simple string of characters is a regular expression that defines one string of characters: itself. A more complex regular expression uses letters, numbers, and special characters to define many different strings of characters. A regular expression is said to match any string it defines.

7.1. Simple Strings

Regular Expression	Matches	Examples
ring	ring	ring,spring, ringing
or not	or not	or not, poor nothing
Thursday	Thursday	Thursday,Thursday’s

7.1.1. Exercise

Try describing what the following command does ?

cat | grep -o "or not"

Try using it to test some example input.

Try using it to test the other regular expressions in the above preceding table.

7.2. Special Characters

Note - in what follows, a regular expression always matches the longest possible string starting as far forward in the line as possible

7.2.1. Period

Regular Expression	Matches	Examples
/ .alk/	stings containing a space, then any character, followed by awk	will talk may balk
/.ing/	all strings with any character preceding ning	singing, ping, before inglenook

7.2.2. Square Brackets

Regular Expression	Matches	Examples
[bB]ill	b or B followed by ill	bill,Bill,billed
t[aeiou].k	t followed by lower case vowel, any character, and k	talk, talkative
number [6-9]	number followed by a space and a single number in range 6-9 (inclusive)	number 7, knumber 601
[^a-zA-Z]	any character that is not a letter	1, pdf2tif

Notice the negating action of ^ when it is inside [].

7.2.3. Asterix

Regular Expression	Matches	Examples
ab*c	a followed by zero or more bs followed by c	ac, abc, aaabc,ac
ab.*c	ab followed by zero or more other characters followed by c	abc,abxc, cat, 756.345, x
t.*ing	t followed by zero or more other characters followed by ing	thing, ting, I was going
[a-zA-Z]*	a string composed of only letters (upper or lower case)	1!!, !any text string
(.*)	as long a string as possible between ()	Get (this (and) that) done
([^)]*)	the shortest string possible between ()	(Get (this or)

7.2.4. Carat (^) and Dollar ($)

Regular Expression	Matches	Examples
^T	a T at the beginning of a line	Time, In Time
^+[0-9]	a + followed by a number at the beginning of a line	+514, +3.14
:$	a : that ends a line	… following :

7.2.5. QuotingSpecial Characters

Regular Expression	Matches	Examples
end.	strings containing end.	end.,end
\$	a dollar sign	lot $ of money
[[0-9]]	a number in []	[9],[90]

7.2.6. Full Regular Expressions

To test these examples you will need to use

cat | egrep -o "or not"

Regular Expression	Matches	Examples
ab+c	a followed by one or more bs followed by a c	yabcw, abbc57
ab?c	a followed by zero or one b followed by a c	back,abcdef
(ab)+c	one or more ab followed by a c	zabcd, abababc
(ab)?c	zero or one ab followed by c	xc, abcc
ab|ac	either ab or ac	ab,ac,abac
(D|N).Jones	D.Jones or N.Jones	N.Jones, P.D.Jones

7.3. Further Special Characters

7.3.1. Standrd Special Characters

These work with grep

Special Character	Function
.	Matches any single character
[xyz]	Defines a character class that matches x,y, or z
^xyz]	Defines a character class that matches any character exceptx,y, or z
[x-z]	Defines a character class that matches any character x through z inclusive
*	Matches zero or more of the preceding character
^	Forces a match to the beginning of the line
$	A match to the end of the line
|	Used to quote special characters
(xyz)	Matches xyz
<	Forces match at beginning of word
>	Forces match at end of word

7.3.2. Full Special Characters

These work with egrep

Special Character	Function
+	Matches one or more occurrences of the preceding character
?	Matches zero or more occurrence of the preceding character
(xyz)+]	One or more occurrences of xyz
(xyz)+	Zero or one occurrence of xyz
(xyz)?	Zero or more occurrences of xyz
(xyz)*	Either xyz or abc
xyz|abc	Either xyc or abc