Perl

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Perl Regular Expressions I

• CSCI 2467 Spring 2007
• Instructor Michael Ruth
• Computer Science Department
• University of New Orleans
• mruth_at_cs.uno.edu

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Any Questions Before We Move On?

• James H. Clark
• Co-founder of
• Netscape communications
• Silicon Graphics Inc
• BS MS in Physics from UNO
• An Important Point
• We went through Shell Programming somewhat
  quickly
• Lets discuss this speed, and they why
• I can never seem to remember all of your
  questions
• please send them to mevia email
• Any other questions before we move on?

3
Topics

• Motivation
• Perl, why we need it
• Very Short History
• Perl mechanics
• Basics
• Execution
• Comments
• Hello World!
• Variables
• Scalars
• Arrays and Hashes
• Control structures
• Selection
• Looping

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Perl, why perl?

• The language is intended to be practical (easy to
  use, efficient, complete) rather than beautiful
  (tiny, elegant, minimal).
• This, as we mentioned earlier, eases our
  prototype needs
• Perl has many and varied applications, compounded
  by the availability of many standard and
  third-party modules
• Used to write CGI programs (web scripting
  Amazon)
• Perl is often used as a glue language, tying
  together systems and interfaces that were not
  specifically designed to interoperate,
• and for "data munging", ie. converting or
  processing large amounts of data for tasks like
  creating reports
• Perl is also widely used in finance and
  bioinformatics, where it is valued for rapid
  application development and deployment, and the
  ability to handle large data sets
• Additionally, perl is a very powerful scripting
  language and one I am very comfortable with
• There are many other powerful scripting languages
  (Python, Tcl/Tk, Ruby, etc)

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The History of perl

• Larry Wall, while working as a programmer at
  Unisys, released it on December 18, 1987
• The name
• Perl was originally named "Pearl", after the
  Parable of the Pearl.
• Wall discovered the existing PEARL programming
  language before Perl's official release and
  changed the spelling of the name.
• Practical Extraction and Report Language is a
  backronym

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Well, what is perl?

• Perl is a general-purpose programming language
  originally developed for text manipulation and
  now used for a wide range of tasks including
• system administration,
• web development,
• network programming,
• GUI development,
• Its major features include
• support for multiple programming paradigms
• Procedural (like C)
• Object-oriented (like Java)
• Functional (like Lisp)
• automatic memory management
• built-in support for text processing
• large collection of third-party modules
• Perl is free and has a very extensive user
  community
• JAPH
• The Obvuscated Perl Contest

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Obfuscated Perl Contest (JAPH)

• What does the following do?

_at_Psplit//,".URRUU\c8R"_at_dsplit//,"\nrekcah xinU
/ lreP rehtona tsuJ"sub p _at_p"rp","up"(P,P)
pipe"rp","up"p(q2)f!forkmapPP
ford (p_)6p_/ P/ix?Pclose_keys
ppppppmapp_/P./ close_pwait
until?map/r/lt_gtp_dqsleep
rand(2)if/\S/print
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Perl Design

• The design of Perl can be understood as a
  response to three broad trends in the computer
  industry
• falling hardware costs
• rising labor costs
• improvements in compiler technology.
• Many earlier computer languages, such as C, were
  designed to make efficient use of expensive
  computer hardware.
• In contrast, Perl is designed to make efficient
  use of expensive computer programmers
• Perl has many features that ease the programmer's
  task at the expense of greater CPU and memory
  requirements
• Tim Toady

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Perl mechanics I

• Some Basics
• Execution
• Comments
• Hello World
• Variables
• Scalars
• Arrays
• Hashes
• Control structures
• Selection
• Looping

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Perl Basics

• Execution
• Using sha-bang mechanism
• Using ltintepretergt ltscriptgt mechanism
• Comments
• everything past the pound is a comment
• Hello world

!/usr/bin/perl -w print (Hello, World!\n)
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Variables

• Scalars
• Simplest form of data (simple data type) that
  perl manipulates
• Can be a number or a string
• Treated identically
• Arrays and List Data
• A list is ordered scalar data
• An array is a variable which holds a list
• Arrays can have any number of elements 0
• Hashes
• Collections of scalar data with elements selected
  by some index value
• Uses the notion of key (scalar) to look up the
  value associated with that key
• AKA Associative arrays

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Scalar Data (Numbers)

• All numbers use the same format internally
• Perl only uses double precision floating point
  values
• This means there are NO integers
• Literals
• Float/Double Literals
• 1.25 1 and a quarter
• 7.25e45 7.25 x 10 to the 45th power
• -6.5e-24 -6.5 x 10 to the -24th power
• Integer Literals
• 12
• -2004
• Octal
• 0ltnumgt
• Hex
• 0xltnumgt or 0Xltnumgt

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Scalar Data (Strings)

• Strings are sequences of characters (length 0 ?)
• Literal Representation
• Single Quotes (not variable interpolated no
  control characters)
• hello
• hello\n
• Double Quotes
• Hello world!\n
• Some of the more important control characters
• \n ? newline
• \r ? return
• \t ? tab

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Operators (numeric)

• Perl provides addition, subtraction, division,
  multiplication and modulus operators
• (,-,/,,)
• Additionally, perl provides an exponential
  operator
• 23 2 to the third power
• Lastly, the comparison operators operate exactly
  as expected
• (lt, lt, , gt, gt, !)
• Return either true or false

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Operators (strings)

• Concatenation uses the . operator
• EX string1 . string2 ? string1string2
• String comparison operators are
• eq, ne, lt, gt, le, ge
• String repetition operator (x)
• EX red x 3 ? redredred
• EX2 5 x 3 ? 555

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Cross-Pollination

• If a string value is used as an operand for a
  numeric operator (say ) it is automatically
  converted to a numeric number (no warning)
• Trailing nonnumerics and leading whitespace are
  ignored
• If a numeric is given where a string value is
  needed (say x) the numeric value is converted
  automatically to its string equivalent
• In other words, you dont have to worry about the
  variable being a scalar or string, perl will take
  care of it for you

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Scalar Variables

• Just as in our shell scripts
• ltvarnamegt
• Assignment Operators
• .
• Auto increment/decrement (must be a scalar
  variable)
• variable variable
• variable-- --variable

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Some Simple Quick Commands

• chop
• chomp
• ltSTDINgt
• Interpolation
• Output
• undefined

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chop/chomp

• chop(ltinputgt)
• Takes a single argument (scalar variable) and
  removes the last character from the string value
  of that variable
• EX var suey chop(var)
• var becomes sue
• chomp(ltinputgt)
• Takes a single argument (scalar variable) and
  removes only newlines from the string value of
  that variable
• EX var Hello World\n chomp(var)
• var becomes Hello World

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ltSTDINgt as a Scalar Variable

• Every time you use the ltSTDINgt in a place where a
  scalar variable is expected, perl reads the next
  complete line from standard input
• The string returned from ltSTDINgt normally has a
  newline on the end (must use chomp)
• EX chomp(a ltSTDINgt)

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Interpolation of scalars in strings

• When a string literal is double quoted it is
  subject to variable interpolation
• Ex
• a fred ? fred
• b a ? fred

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Output using print

• How do we get things out?
• The print function takes the values within
  parenthesis and outputs them on standard output
• For example
• print hello world\n
• print(hello world\n)

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The Undefined Value

• What happens when you use a variable before its
  defined
• You get a undef value
• Treated like zero when used numerically
• Treated like empty string when used as a string
• Normally, ltSTDINgt returns the last line read in,
  but if there are no more lines to read it returns
  the undef

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Lists and Arrays

• Literals
• Variables
• Operators
• Functions
• push/pop
• shift/unshift
• sort
• chomp
• ltSTDINgt

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Array Literals

• Literal Representation
• (1,2,3)
• (fred,2,3)
• ()
• List constructor operator
• (1 .. 5) same as (1, 2, 3, 4, 5)
• Quote Word
• A shortcut function
• qw(1 2 3 4 5)

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Array Variables

• Array variables holds a single list value
• Sigil is _at_ rather than
• EX _at_fred
• The value of an unassigned array is ()
• Expressions can refer to array variables as a
  whole, or it can examine and modify individual
  elements of the array

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Array Assignment

• Some examples
• _at_fred (1, 2, 3)
• _at_barney _at_fred
• _at_fred (1, 2, _at_barney) ? (1, 2, 1, 2, 3)
• If a scalar is assigned, then the scalar becomes
  the only element of the list
• _at_fred 1 is equivalent to _at_fred (1)
• If an array variable is assigned to a scalar, we
  get the size of the array
• size _at_fred

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Array Element Access

• All arrays in perl begin with 0
• The form is
• ltarraynamegtltindicegt
• Examples
• _at_fred (7, 8, 9)
• fred0 ? 7
• (fred0, fred1) (fred1, fred1)
• Array Slicing
• Accessing a list of elements from the same array
  is called a slice
• EX
• _at_fred0,1 ? (fred0, fred1)
• _at_fred0,1 _at_fred1,0
• _at_fred0,1,2 _at_fred1,1,1
• _at_fred0,1 (9,10)

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Array Element Access II

• The indices do not have to be literal integers
• Example
• a 2
• freda equivalent to fred2
• Can even use arrays as indices (slices)
• Example
• _at_barney (0,1,2)
• _at_fred(_at_barney)
• If you access a variable beyond that of the array
  undef is returned w/o warning
• If you set a variable beyond that of the array
  the array is simple extended
• You can use fred to get the last index (you can
  also set it)
• Lastly, a negative subscript counts backward from
  the end

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Other Array Methods

• push/pop
• Pop removes and returns the last element of the
  array
• FM pop(varlist)
• Push accepts a list and a list of values to be
  pushed onto the end of the array
• FM push(varlist,list)
• shift/unshift
• Same as push/pop except that it looks at the
  beginning
• reverse
• Reverses the order of the elements
• sort
• Sorts the array (string)
• chomp
• Removes the \n from the end of all the strings
  in the arrays

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Array and ltSTDINgt

• All remaining lines up to the end of the file and
  then each line becomes an element of the array
• _at_fred ltSTDINgt
• The last and empty line is not added to the array

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Hashes AKA Associative Arrays

• Hash Variables
• Literals
• Variables
• Functions
• Keys
• Values
• Each
• Delete
• Hash Slices

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Hash Variables and Literals

• Hash variables are of the form
• ltvariable namegt
• Hashes are almost never accessed as a whole, they
  are normally accessed thought their variables
• Elements are referenced with
• ltvariablenamegtkey where key is any scalar
  expression
• Ex fred fredmike A
• Literal representation of a hash
• There is no literal representation of a hash
• You can unwind arrays to hashes and vice versa

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Hash Functions

• keys(hash) function yields a list of all the
  keys in the hash
• In a scalar context, it yields the number of
  elements in the hash
• values(hash) function yields a list of all the
  elements in the hash
• each(hash) function is used to iterate over
  every element in the hash
• Returns key-value pair until empty
• EX
• while ((first, last) each(hash))
• print The last name of first is last\n
• delete fredmike removes the element
  referenced by the key mike

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Hash Slices

• Just like arrays we can get slices of hashes
• Assume
• scorefred 205
• scorebarney 300
• scorewilma 195
• _at_scorefred,barney,wilma (205,300,195)

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Control Structures

• Basic issues
• If/unless
• While/until
• Do while/until
• For
• Foreach

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Control Structures Basics

• Statement Blocks
• We block off groups of statements using brackets
• BLOCK
• Conditionals are evaluated expressions (string)
• False
• The string is empty (length of zero)
• A string consisting of the single character 0
• True
• Everything else!!!

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If/unless

• IF
• if (condition)
• Statement list
• elsif (condition)
• Statement list
• else
• Statement list
• UNLESS
• unless (condition)
• Statement list
• else
• Statement list

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while/until

• WHILE
• while (condition)
• Statement list
• UNTIL
• until (condition)
• Statement list

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do while/until

• do while
• do
• Statement list
• while condition
• do until
• do
• Statement list
• until condition

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for/foreach

• FOR
• for (initial_exp test_exp re-init_exp)
• Statement list
• FOREACH
• Takes a list and iterates through it
• foreach identifier (_at_list)
• Statement list
• Use the identifier variable inside the statement
  list
• Example
• for each b (_at_fred)
• print b

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Resources

• Learning Perl
• The Llama Book
• Programming Perl ?
• The Camel Book
• Mastering Regular Expressions
• The Owls Book
• Perl Cookbook
• The Ram Book

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The future

• Perl Regular Expressions II
• Basic I/O
• Regular Expressions
• Functions
• Files
• Access
• Contents
• Directories
• Introduction to C Programming
• Among others a History of C
• More or less follow the book

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Mystic Krewe of Barkus 2/11 _at_ 2pm
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Questions?