Introduction to Perl

1. Introduction to Perl by Liam McNamara Practical Extraction and Report Language High-level general purpose 'scripting' language, used mainly for text processing, administrative tasks, database and network/web programming. Also data manipulation tasks such as in bioinformatics and finance. It's interpreted, procedural and dynamically typed with automated memory management. It's widely deployed on *nix systems distributed with most modern distributions and also available for Windows (ActivePerl project). "duct tape of the Internet"

2. History of Perl Larry Wall is a linguist and the 'benevolent dictator' of the language, he created it while he was a sysadmin at NASA. Version 1.0 was released at the end of 1987, and it is now at v5.8. It was predominately created for source code patching and report generating. The perl distribution is written in C and has been ported to over 100 platforms. Behaviour and syntax influenced by: awk, sed, C, shell script and even some lisp It's free software licensed under the GNU General Public License.

3. Overview of the language • Dynamically typed making it flexible • Native regular expressions • 'Interpreted', allowing fast development cycles • Modular and Object Orientated'ish • Provides automated security checks • Very expressive - problem orientated • Arguably ugly? Its been described as a 'write only language' • Language is currently defined by the implementation but • the next version is currently being specified first. • 'Makes easy things trivial and hard things possible'

4. Simple Example Mandatory Hello world A perl program is a list of commands terminated by semi-colons. Often used as 'one-liners' as part of shell scripts. $ perl -e 'print “Hello World\n”;' $ perl -e 'print “$> $$”;' #print PID UID Variables are denoted by using sigils, this special character ($@%& or *) shows the type (like in BASIC). This facilitates implicit type conversion which speeds development but is sometimes confusing as a lot can be going on behind the scenes. $scalar = '4'; # the ASCII character $scalar++; # increment print $scalar + “\n”; # prints '5' with newline Example 1

5. Arrays, Hashes and Refs All mod-cons Array- Numbered list of data items, will automatically increase size. @array = ('a', 'b', 'c'); print $array[0]; #will print 'a' Hash- An associative list, mapping scalar keys to values. %hash = ('key1' => 'value1', 'key2' => 'value2'); $hash{'key3'} = 'value3'; Reference- Similar to C pointer it's simply a scalar containing the address of another variable, which could be another scalar, array, hash, code block or even another reference. $ref = \@array; print $ref->[0]; $coderef = sub {print “howdy doody\n”}; &$coderef; #will perform print

6. TMTOWTDI There's More Than One Way To Do It Blessing or curse? Although this allows greater expressiveness when quantifying a problem, it can reduce readability for other programmers (and sometimes yourself at a later date). All of these do the same: if ($x == 0) {$y = 10;} else {$y = 20;} $y = $x==0 ? 10 : 20; $y = 20; $y = 10 if $x==0; unless ($x == 0) {$y=20} else {$y=10} if ($x) {$y=20} else {$y=10} $y = (10,20)[$x != 0];

7. Syntactical Sugar Range operator: 1..10 # equates to the list (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) Default variables are specially named variables whose content depends on the enclosing block: $_ -variable being iterated over @_ -the parameters to a subroutine Readline operator will read a line from an open file, or by default the STDIN handle: <FileHandle> or <> Backticks will perform a system call and return the result: $daemons = `cat /etc/passwd| grep daemon`

8. Flow Control C style for for (initialise; exit-condition; per-loop) { ... } Scripting style for Iterate over the given '@list', setting '$_' to each data item in the list. foreach (@list) { print $_; #don't even need the '$_' here ... } Straight-forward while while (expression) { ... } do { ... } while (expression); Example 2

9. Subroutines Informal parameters Parameters are passed as a flattened list, merging all into one large array called @_. This is defined for the lexical scope of the subroutine block and can be accessed by 'shift' (which will default to 'shifting' the first element off @_). They also employ context awareness of their calling allowing different behaviour depending if a scalar is wanted or not, using the builtin command WANTARRAY. # adduser('someguy','secret',('users','wheel')) sub adduser ($$@) { $username = shift; $password = shift; @groups = @_; ... }

10. Regex Regular Expressions Matching /pattern/ - tests if a pattern matches the input if ($username =~ /admin/) { doAdmin(); } . - matches any single character except newline \s -match any whitespace character a* - matches 0-∞ amount of 'a's b{1,4} - matches 1-4 occurrences of 'b' Grouping () - allows naming of matched sections 'bookmark.html' =~ /(.*)\.(.*)/; $1 # 'bookmark' $2 # 'html' Character class [abcd] – matches one of a,b,c or d The regex engine will find any possible way of matching the pattern to the input, backtracking when a decision leads to a false result.

11. Regex 2 Substitution- s/old/new/ “The old brown fox” becomes “The new brown fox” “bold and old ” becomes “bnew and old” Regular expressions can perform quite complex tasks. Strips any leading whitespace and repeats the first word: s/\W*((\w*).*)/$2 $1/ Tests if a number is prime: perl -wle 'print “Prime” if (1 x shift) !~ /^1?$|^11+$/ number Example 3

12. Object Orientated (nearly) A perl module can be created with special subroutines that allow the module to be instantiated as an 'object'. package objecttype; sub new { $self = {}; #empty hash bless($self); return $self } ... 1; #returned to the 'use' command to show success The 'object' can then be instantiated by using the 'new' keyword use objecttype; $obj = new objectname(); $obj->methodname(); Example 4

13. Tie variables Most OO functionality can be replicated with a much cleaner feature of perl, Tie. An OO style module is defined with special sub names, this can then be bound to a variable (scalar, array or hash). When that variable is consequently used the subs of the module are called. Specially named subs need to be defined depending on the variable type that the 'class' is going to be tied to. $scalar - TIESCALAR, FETCH, STORE @array - TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE, SHIFT.. %hash - TIEHASH, FETCH, STORE, FIRSTKEY, NEXTKEY, EXISTS... This feature provides many OO benefits through the natural interface of variables, avoiding the awkward perl OO syntax; Example 5

14. XS Extension interface between Perl and C An XSUB provides a layer of abstraction between the Perl engine some C code. The interface definition written in XS allows perl code to call C functions, obviously providing a performance improvement (useful for computational bottlenecks). The XS interface stubs are combined with the C library by the special compiler xsubpp and allows the perl code to utilise it. The extra code allows the necessary context switches making all parameters and global variables available to the C functions. An empty perl module has to serve as a dummy module to allow the bootstrapping of the C code. The C library is then linked to the perl runtime library.

15. POD- Embedded Docs Plain Old Documentation POD allows the documentation to be embedded in the code, allowing them to be distributed together, and hopefully promote good commenting. =head1 Main-Header-Text Documentation with B<Bold Text> =over 4 =item and indented sections or 'items' =back =cut There are tools to extract the POD and create different file types pod2latex, pod2html, pod2man. Example 6

16. CGI- Web coding One of the major reasons perl has become one of the main languages of the web is its quick development time (automatic memory management, dynamic typing, interpreted). Advanced text (HTML,XML) processing, focus by the community on related tasks providing a lot of web/network related development. It's a key feature of 'platforms' such as LAMP (Linux+ Apache+ MySQL+ perl) for web development. use CGI; mod_perl is a module of the Apache web server, which provides the ability for the web server to understand perl and avoids spawning a separate process, caches memory and provides persistent database connections between separate HTTP requests.

17. Community comp.lang.perl Usenet – CPAN Comprehensive Perl Archive Network There is a well developed community surrounding the perl language, providing an extensive resource of testers and developers for the distribution and 3rd party code projects. CPAN is a huge repository (9144+ projects) of perl code providing modules to perform a wide selection of tasks, so you can avoid 'reinventing the wheel'. There are Obfuscated Perl Contests celebrating the non-alphanumeric syntax and general unreadability. They are marked on brevity, creativity and power. Perl Mongers is an international collection of perl user groups. The most recent meet for London.pm was last month. Example 7

18. Future of Perl Perl6 • Perl is currently undergoing a complete rebuild of the language and internals, there is a fully specification being specified first. It will not be keeping backward compatibility in an effort to clean-up many historical features. Adding ideas such as optional static typing, Unicode operators and formal parameters. • Parrot VM features • Generation of an intermediate bytecode form (like python) for quick re-execution • Native binary format creation possible • Register based VM (JVM is stack based) • Will be coded in itself. • There is also PUGS, an implementation of the Perl6 engine in Haskell which will eventually be used to translate itself.

19. References • http://perl.com - main site • http://cpan.org - massive collection of Perl modules • http://parrotcode.org – new Perl6 VM • http://activestate.com/ActivePerl/ - Windows Perl • http://macperl.com – Perl for old Mac OS • http://pm.org – perlmongers • http://perl.apache.org- mod_perl • $ man perl • $ man perldoc Questions?