Practical Extraction

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Practical Extraction Report Language
PERL
Joseph Beltran
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What is PERL?

• An interpreted language that is optimized for
  I/O, system tasks and string manipulation
• Larry Wall originally created PERL because he
  sought the need for a language that combines the
  best features of other scripting languages

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Uses of PERL

• Text Processing
• can manipulate textual data, email, news
  articles, log files, or just about any kind of
  text, with great ease
• System Administration
• particularly useful for tying together lots of
  smaller scripts, working with file systems,
  networking, so on
• CGI and Web Programming
• can be used to process and generate HTML
• Other Uses
• DNA sequencing for The Human Genome Project
• NASAs Satellite Systems Control
• Perl Data Language for number-crunching
• Perl Object Environment for event-driven machines

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Variables

• PERL provides three kinds of variables
• Scalars, Arrays, and Associative Arrays
• The initial character of the name identifies the
  particular type of variable and, hence, its
  functionality.

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Variables

• Scalar Variables
• name
• Strings and numbers whether integers or decimals
  are treated in the same way
• aVar 4
• bVar 4.5 a decimal number
• cVar 3.14e10 a floating point number
• dVar "a string of words
• eVar aVar . bVar note use of . to
  concatenate strings

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Variables

• Arrays
• _at_name()
• Single dimension list of scalars
• _at_aList (2, 4, 6, 8) explicit values
• _at_aList (1..4) range of values\
• _at_aList (1, "two", 3, "four") mixed values
• _at_aList () empty list
• aList index of last item
• aList0 first item in _at_aList

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Variables

• Associative Arrays
• name
• A two-dimensional array, for use with
  attribute/value pairs.
• The first element in each row is a key and the
  second element is a value associated with that
  key.
• aAA"A" 1 creates first row of associative
  array
• aAA"B" 2 creates second row of
  associative array
• aAA ("A", 1, "B", 2) same as first two
  statements

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Operators

• If variables are the nouns, PERL provides
  operators, which are the verbs.
• Operators access and change the values of
  variables.
• Some assignments apply to all three kinds of
  variables. However, most are specialized with
  respect to their types.

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Operators

• Numeric Operators
• plus
• - minus
• multiply
• / divide
• exponentiation
• modulus
• equal
• ! not equal
• lt less than
• gt greater than

• lt less than or equal to
• gt greater than or equal to
• binary assignment
• - same, subtraction
• same, multiplication
• auto increment
• -- auto decrement

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Operators

• Literal Operators
• . concatenate
• x n repetition e.g., "A" x 3 gt "AAA"
• eq equal
• ne not equal
• lt less than
• gt greater than
• le less than or equal to
• ge greater than or equal to

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Control Structures

• PERL is an iterative language in which control
  flows from the first statement in the program to
  the last statement unless something interrupts.
• Some of the things that can interrupt this linear
  flow are conditional branches and loop
  structures.

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Control Structures

• If Conditional Statement
• if (expression_A)
• A_true_stmt_1
• A_true_stmt_2
• elseif (expression_B)
• B_true_stmt_1
• B_true_stmt_2
• else
• false_stmt_1

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Control Structures

• While Loop Statement
• LABEL while (expression)
• stmt_1
• stmt_2

• Until Loop Statement
• LABEL until (expression)
• stmt_1
• stmt_2

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Control Structures

• For Loop Statement
• LABEL for (initial exp test exp increment
  exp)
• stmt_1
• stmt_2

• For Each Loop Statement
• LABEL foreach i (_at_aList)
• stmt_1
• stmt_2

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Input / Output

• PERL uses filehandles to control input output
• These are STDIN for accessing input, STDOUT for
  printing output, and STDERR for writing error
  messages
• Additional filehandles are created by the open
  command

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Input / Output

• Opening Files
• Syntax open (FILEHANDLE, "filename")
• Examples
• open (INPUT, "index.html") for reading
• open (OUTPUT, "gt index.html") for writing
• open (OUTPUT, "gtgt index.html") for appending
• Closing Files
• Syntax close (FILEHANDLE)
• Example
• close (INPUT)

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Regular Expressions

• Regular expressions give us extreme power to do
  pattern matching on text documents.
• Patterns
• Literal String Pattern
• if (/cat/) print "cat found in a"
• Single-Character Pattern
• /.at/ matches "cat, and "bat
• /0-9/ matches 0 to 9
• /0123456789/ matches 0 to 9

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Regular Expressions

• Operators
• Substitution
• s/cat/dog/ replaces "cat" with "dog
• s/cat/dog/gi same but ignores case
• Splitting
• _at_a split(/cat/, a) removes cat from
  a
• Joining
• a join (cat", "dog", "bird") returns
  "catdogbird"

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Examples

• Example 1
• STDIN and STDOUT, Looping and Conditions
• Example 2
• SEARCH and REPLACE strings
• Example 3
• FILE READING and WRITING
• Sample scripts are run using Active Perl 5.6
  
• from www.activestate.com