Regular Expressions

1
Regular Expressions

• What is it?
• Simple string
• Special Characters

2
What is it?

• It is a method of defining a set of one or more
  strings of characters.
• Linux utilities such as vi, grep, awk, sed, use
  regular expression to search for and replace
  strings.
• A regular expression matches any string it
  defines.

3
Simple String

• A simple string matches only itself.
• Example
  1. regular expression /ring/ matches
  ring, spring, ringing, stringing
  2. /or not/ matches or
  not, poor nothing 3. /Thursday/ matches
  Thursday, Thursdays

4
Special Characters

• We can use special characters within a regular
  expression to cause the regular expression to
  match more than one string.
• Some examples of special characters are period,
  brackets, asterisk, caret, and dollar sign.

5
Period (.)

• A period matches any character.
• Example
  1. / .alk/ matches all
  strings consisting of a SPACE followed by any
  character followd by alk such as will talk, may
  balk
• 2. /.ing/ matches all strings consisting of
  any characters preceding ing such as singing,
  ping

6
Square Brackets

• defines a character class that matches any
  sing character within the brackets.
• Examples
  /bBill/ gt bill, Bill, billed
  /taeiou.k/gttalkative, stink, teak, tanker
• Note! If the first character following the left
  square bracket is a caret (), it matches any
  single character not within the brackets.
  
  /a-zA-Z/ matches any
  character that is not a letter gt 1, 7, _at_, .,
  , !

7
Asterisk

• An asterisk represents zero or more occurrences
  of the preceding regular expression.
  
  1. /abc/ a followed by zero or more bs
  followed by a c gt ac, abc, abbc, dcabbc
  
• 2. /a-zA-Z / matches a string composed only
  of letters and spaces gt any string without
  numbers or punctuation.

8
Period and Asterisk (.)

• An asterisk following a period matches any string
  of characters.
  1. /ab.c/ matches ab followed
  by zero or more characters followed by c. gtabc,
  abxc, ab45c, xab 756.345 x cat
• 2. /t.ing/ macthes t followed by zero or more
  characters followed by ing gtthing, ting, I
  thought of going
  
  3. /(.)/ matches as long a string as
  possible between ( and ). gt (this), (that)

9
Caret

• A regular expression that begins with a caret can
  match a string only at the beginning of a line.
  
• 1. /T/ matches a T at the beginning of a
  line gt This line , That Time,
  2. /0-9/ matches a
  plus sign followed by a digit at the beginning of
  a line gt 5 79879, 759 keep this .

10
Dollar Sing

• A dollar sign at the end of a regular expression
  matches the end of a line.
• 
  // matches
  a colon that ends a line gt .below

11
Quoting Special Characters

• We can quote any special character (but not a
  digit or a parenthesis) by preceding it with a
  backslash.
• Remember! Quoting a special character makes it
  represent itself.
• 1. /end\./ gt The end., send.,
  pretend.mail
  2. / \\
  / gt A single back slash \
  3. / \/ gt An asterisk

12
Task

• What does the regular expression of /Th.is/
  matches when applied to the following text
• This (rug) is not what it once was ( a long time
  ago), is it?