Perl

Control Structures

A statement block is a sequence of Perl statements enclosed in matching curly braces:

{
    first_statement;
    second_statement;
    …
    last_statement;
}

A statement block may be used in place of a single statement, but not vice versa!

Conditional Statements

In conditional statements, Perl interprets an expression as either true or false.

• False expressions evaluate to
• undef
• ""
• "0"
• 0
• True expressions are everything else

if / unless statements

An if statement executes the statement block only if the expression evaluates to true

• if ( expression )
      statement_block
• if ( expression )
      statement_block
  else
      statement_block
• if ( expression )
      statement_block
  elsif ( expression )
      statement_block
  else
      statement_block

An unless statement executes the statement block only if the expression evaluates to false

• unless ( expression )
      statement_block
• unless ( expression )
      statement_block
  else
      statement_block

while / until statements

A while statement repeatedly executes the statement block, as long as the expression evaluates to true

while ( expression )
    statement_block

An until statement repeatedly executes the statement block, as long as the expression evaluates to false

until ( expression )
    statement_block

do {} while / until statements

A do {} while statement executes the statement block, then reiterates as long as the expression evaluates to true

do
    statement_block
while ( expression );

A do {} until statement executes the statement block, then reiterates as long as the expression evaluates to false

do
    statement_block
until ( expression );

for / foreach statements

A for statement will

1. execute the initial expression
2. execute the statement block if the test expression evaluates to true
3. execute the update expression, then reiterate if the test expression still evaluates to true

for (init_exp; test_exp; update_exp)
    statement_block

A foreach statement will

• assign elements (sequentially) from a list to a scalar index variable
• execute the statement block once for each given value in the list

foreach scalar_index ( list )
    statement_block

Hashes

A hash is an associative array that uses scalar keys to access array elements

• A hash variable name begins with % followed by a letter, then more letters, numbers, and underscores
• Each element of the hash is a scalar variable accessed by a string index (key)
• Elements may be added either one at a time or all at once
• $birthday{"Mary"} = "12/10/81";
• %birthday = ("Mary", "12/10/81", "John", "8/6/79");
• Actual order of the elements in the hash is determined by the system

Hash Functions

• reverse swaps keys and values
  %people = reverse %birthday;
• keys yields a list of all key values in the hash
  @names = keys(%birthday);
• values function returns a list of all the current values of the hash
  @dates = values(%birthday);
• each will select every key/value pair
  while (($n,$d) = each(%birthday)) { print "$n's birthday is $d\n"; }

Regular Expressions in Perl

Perl uses regular expressions -- usually delimited by / / -- to match a template to a string. Some patterns are like those available with sed and the Korn shell.

• literal strings (case sensitive)
  /abc/ literally matches the sequence "abc"
• single character selection
  /./ matches any character except newline
  /[ab0-9]/ matches "a" or "b" or some digit
• multipliers
  /a*b+c?/ matches 0 or more "a" followed by 1 or more "b" followed by 0 or 1 "c"
• beginning and end of line
  /^A/ matches a string that begins with "A"
  /ing$/ matches a string that ends with "ing"

Matching with Regular Expressions

• Matching
• $a =~ /the/
  true if variable $a contains pattern the
• /the/
  true if variable $_ contains pattern the
• Substitution
• $a =~ s/abc/xyz/
  replaces 1st occurrence of pattern abc with xyz in variable $a
• s/abc/xyz/
  replaces 1st occurrence of pattern abc with xyz in variable $_
• s/abc/xyz/g
  replaces all occurrences of pattern abc with xyz in variable $_

Split & Join

These operators break a string into fields, and glue fields together in a single string

• Split uses a regular expression as a field separator
• @fields = split(/[:,]/, $line);
  uses either ":" or "," as a field separator
• Join uses a given string as a field separator
• $longlist = join(":", @fields);
  recombines elements of array @fields into a string, with elements separated by ":"

---

Common Gateway Interface

A CGI program is a program run by the Web server in response to a request from a client. Examples are

• Forms: gather information sent by the client
• Gateways: provide web access to information on the server, such as a database
• Virtual documents: generate customized web pages on the fly

All CGI programs must print one of 3 headers, followed by a blank space, to standard output:

• Content-type indicates http will follow
  Content-type: text/html
• Location points to a document or other valid URL
  Location: http://www.perl.com/CPAN
• Status doesn't affect what the browser shows
  Status: 204 No Response

Creating a Form

A form begins with a <FORM> tag and ends with a </FORM> tag.

• Attributes of the form include
• ACTION specifies URL of the CGI program
• METHOD indicates where parameters will go: GET (default) or POST (standard input)
• <FORM METHOD=GET ACTION= "http://www.com/cgi-bin/hi.pl">
• Input fields are indicated with the <INPUT> tag
• TYPE indicates the type of input field
• NAME is a key word representing that field
• VALUE is the default value
• <INPUT TYPE=submit VALUE="Go!">
• Types of input fields include
• TEXT (default)
• CHECKBOX
• HIDDEN
• RADIO
• SUBMIT
• PASSWORD

Processing Input

Arguments are generally passed to the CGI program as part of the URL. The CGI program accesses these arguments by examining environment variables.

• QUERY_STRING contains the parameters
  $form_in = $ENV{'QUERY_STRING'};
• CONTENT_LENGTH = size of QUERY_STRING
  read (STDIN, $query, $ENV{'CONTENT_LENGTH'});
• REQUEST_METHOD is GET or POST
  if ($ENV{'REQUEST_METHOD'} eq "GET")