Words

1
Words

• Words are the building blocks of sentences

2
Regular expressions (in Perl)

• a regular expression is an algebraic notation
  for characterizing a set of strings p. 22
• Regular expressions are used to specify search
  strings.

3
Sequences of characters

• Matching a sequence of characters
• //
• Examples
• /a/ matches the character a
• /fred/ matches the string fred
• Note
• /fred/ does not match Fred!

4
Disjunction

• Square brackets are used to indicate disjuction.
• Examples
• /Ff/ matches either f or F
• /Ffred/ matches either fred or Fred

5
Ranges

• Sometimes it is useful to specify any digit or
  any letter.
• Any digit can be written as /0123456789/
• An alternative is to use a special range
  notation /0-9/
• Any letter can be specified as /A-Za-z/

6
Negation

• To search for the opposite of a pattern, use the
  caret () in front of the pattern, enclosed in
  square brackets.
• Examples
• /a/ matches anything except a
• /0-9/ matches anything except a digit
• N.B. Every line ends with a newline (\n)!

7
Matching 0 or 1 occurrence

• The ? matches zero or one occurrences of the
  preceding expression.
• Examples
• /a?/ matches a or (nothing)
• /cats?/ matches cat or cats

8
The Kleene star and plus

• The Kleene star () matches zero or more
  occurrences of the preceding expression.
• Examples
• /a/ matches , a, aa, aaa, etc.
• /ab/ matches , a, b, aa, ab, ba,
  bb, etc.
• matches one or more occurrences

9
Wildcard

• The period (.) matches any single character
  except the newline (\n).

10
Anchors

• Anchors are used to restrict a match to a
  particular position within a string.
• anchors to the start of a line
• anchors to the end of a line
• \b anchors to a word boundary
• \B anchors to a non-boundary

11
Conjunction

• Two regular expressions are conjoined by
  juxtaposition (placing the expressions side by
  side).
• Examples
• /a/ matches a
• /m/ matches m
• /am/ matches am but not a or m alone

12
Disjunction

• We have already seen disjunction of characters
  using the square bracket notation
• General disjunction is expressed using the
  vertical bar (), also called the pipe symbol.