Perl BasicsPerl Basics

A Perl Tutorial

NLP Course - 2006

What is Perl?What is Perl? Practical Extraction and Report Language Interpreted Language

Optimized for String Manipulation and File I/O Full support for Regular Expressions

Running Perl ScriptsRunning Perl Scripts Windows

Download ActivePerl from ActiveState Just run the script from a 'Command Prompt'

window UNIX – Cygwin

Put the following in the first line of your script#!/usr/bin/perl

Run the script% perl script_name

Basic SyntaxBasic Syntax Statements end with semicolon ‘;’ Comments start with ‘#’

Only single line comments Variables

You don’t have to declare a variable before you access it

You don't have to declare a variable's type

Scalars and IdentifiersScalars and Identifiers Identifiers

A variable name Case sensitive

Scalar A single value (string or numerical) Accessed by prefixing an identifier with '$' Assignment with '='

$scalar = expression

StringsStrings Quoting Strings

With ' (apostrophe) Everything is interpreted literally

With " (double quotes) Variables get expanded

With ` (backtick) The text is executed as a separate process, and

the output of the command is returned as the value of the string

Check 01_printDate.pl

String Operation Arithmetic

lt less than <

gt greater than >

eq equal to ==

le less than or equal to <=

ge greater than or equal to >=

ne not equal to !=

cmp compare, return 1, 0, -1 <=>

Comparison OperatorsComparison Operators

Operator Operation

||, or logical or

&&, and logical and

!, not logical not

xor logical xor

Logical OperatorsLogical Operators

Operator Operation

. string concatenation

x string repetition

.= concatenation and assignment

$string1 = "potato";

$string2 = "head";

$newstring = $string1 . $string2; #"potatohead"

$newerstring = $string1 x 2; #"potatopotato"

$string1 .= $string2; #"potatohead"

String OperatorsString Operators

Check concat_input.pl

Perl FunctionsPerl Functions Perl functions are identified by their unique names

(print, chop, close, etc) Function arguments are supplied as a comma

separated list in parenthesis. The commas are necessary The parentheses are often not Be careful! You can write some nasty and unreadable

code this way!

Check 02_unreadable.pl

ListsLists Ordered collection of scalars

Zero indexed (first item in position '0') Elements addressed by their positions

List Operators (): list constructor , : element separator []: take slices (single or multiple element chunks)

List OperationsList Operations sort(LIST)

a new list, the sorted version of LIST reverse(LIST)

a new list, the reverse of LIST join(EXPR, LIST)

a string version of LIST, delimited by EXPR split(PATTERN, EXPR)

create a list from each of the portions of EXPR that match PATTERN

Check 03_listOps.pl

ArraysArrays A named list

Dynamically allocated, can be saved Zero-indexed Shares list operations, and adds to them

Array Operators @: reference to the array (or a portion of it, with []) $: reference to an element (used with [])

Array OperationsArray Operations push(@ARRAY, LIST)

add the LIST to the end of the @ARRAY pop(@ARRAY)

remove and return the last element of @ARRAY unshift(@ARRAY, LIST)

add the LIST to the front of @ARRAY shift(@ARRAY)

remove and return the first element of @ARRAY scalar(@ARRAY)

return the number of elements in the @ARRAY

Check 04_arrayOps.pl

Associative Arrays - HashesAssociative Arrays - Hashes Arrays indexed on arbitrary string values

Key-Value pairs Use the "Key" to find the element that has the

"Value" Hash Operators

% : refers to the hash {}: denotes the key $ : the value of the element indexed by the key (used

with {})

Hash OperationsHash Operations keys(%ARRAY)

return a list of all the keys in the %ARRAY values(%ARRAY)

return a list of all the values in the %ARRAY each(%ARRAY)

iterates through the key-value pairs of the %ARRAY delete($ARRAY{KEY})

removes the key-value pair associated with {KEY} from the ARRAY

Arrays ExampleArrays Example#!/usr/bin/perl # Simple List operations

# Address an element in the list@stringInstruments =

("violin","viola","cello","bass"); @brass =

("trumpet","horn","trombone","euphonium","tuba");

$biggestInstrument = $stringInstruments[3];

print("The biggest instrument: ", $biggestInstrument);

# Join elements at positions 0, 1, 2 and 4 into a white-space delimited string

print("orchestral brass: ", join(" ",@brass[0,1,2,4]), "\n");

@unsorted_num = ('3','5','2','1','4');@sorted_num = sort( @unsorted_num );

# Sort the listprint("Numbers (Sorted, 1-5): ", @sorted_num, "\n");

#Add a few more numbers@numbers_10 = @sorted_num;push(@numbers_10, ('6','7','8','9','10'));print("Numbers (1-10): ", @numbers_10, "\n"); # Remove the lastprint("Numbers (1-9): ", pop(@numbers_10), "\n"); # Remove the firstprint("Numbers (2-9): ", shift(@numbers_10), "\n"); # Combine two opsprint("Count elements (2-9): ",

$#@numbers_10;# scalar( @numbers_10 ), "\n"); print("What's left (numbers 2-9): ",

@numbers_10, "\n");

Hashes ExampleHashes Example#!/usr/bin/perl # Simple List operations

$player{"clarinet"} = "Susan Bartlett"; $player{"basson"} = "Andrew Vandesteeg"; $player{"flute"} = "Heidi Lawson"; $player{"oboe"} = "Jeanine Hassel"; @woodwinds = keys(%player); @woodwindPlayers = values(%player);

# Who plays the oboe?print("Oboe: ", $player{'oboe'}, "\n");

$playerCount = scalar(@woodwindPlayers);

while (($instrument, $name) = each(%player)){

print( "$name plays the $instrument\n" );}

Pattern MatchingPattern Matching A pattern is a sequence of characters to be

searched for in a character string /pattern/

Match operators =~: tests whether a pattern is matched !~: tests whether patterns is not matched

Pattern Matches Pattern Matches

/def/ "define" /d.f/ dif

/\bdef\b/ a def word /d.+f/ dabcf

/^def/ def in start of line

/d.*f/ df, daffff

/^def$/ def line /de{1,3}f/ deef, deeef

/de?f/ df, def /de{3}f/ deeef

/d[eE]f/ def, dEf /de{3,}f/ deeeeef

/d[^eE]f/ daf, dzf /de{0,3}f/ up to deeef

PatternsPatterns

Character RangesCharacter RangesEscape

SequencePattern Description

\d [0-9] Any digit

\D [^0-9] Anything but a digit

\w [_0-9A-Za-z] Any word character

\W [^_0-9A-Za-z] Anything but a word char

\s [ \r\t\n\f] White-space

\S [^\r\t\n\f] Anything but white-space

BackreferencesBackreferences Memorize the matched portion of inputUse of parentheses.

/[a-z]+(.)[a-z]+\1[a-z]+/ asd-eeed-sdsa, sd-sss-ws NOT as_eee-dfg

They can even be accessed immediately after the pattern is matched \1 in the previous pattern is what is matched by (.)

Pattern Matching OptionsPattern Matching Options

Escape Sequence

Description

g Match all possible patterns

i Ignore case

x Ignore white-space in pattern

SubstitutionsSubstitutions Substitution operator

s/pattern/substitution/options If $string = "abc123def";

$string =~ s/123/456/

Result: "abc456def" $string =~ s/123//

Result: "abcdef" $string =~ s/(\d+)/[$1]/

Result: "abc[123]def“ Use of backreference!

Predefined Read-only VariablesPredefined Read-only Variables$& is the part of the string that matched the regular expression

$` is the part of the string before the part that matched

$' is the part of the string after the part that matched

EXAMPLEEXAMPLE$_ = "this is a sample string";/sa.*le/; # matches "sample" within the string# $` is now "this is a "# $& is now "sample"# $' is now " string"Because these variables are set on each successful match, you should save the values elsewhere if youneed them later in the program.

The split and join FunctionsThe split and join FunctionsThe split function takes a regular expression and a string, and looks for all occurrences of the regular expression within that string. The parts of the string that don't match the regular expression are returned in sequence as a list of values.

The join function takes a list of values and glues them together with a glue string between each list element.

Split ExampleSplit Example Join ExampleJoin Example

$line = "merlyn::118:10:Randal:/home/merlyn:/usr/bin/perl";@fields = split(/:/,$line); # split $line, using : as delimiter# now @fields is ("merlyn","","118","10","Randal",# "/home/merlyn","/usr/bin/perl")

$bigstring = join($glue,@list);

For example to rebuilt the password file try something like:$outline = join(":", @fields);

String - Pattern ExamplesString - Pattern ExamplesA simple Example

#!/usr/bin/perlprint ("Ask me a question politely:\n");

$question = <STDIN>;

# what about capital P in "please"?if ($question =~ /please/){

print ("Thank you for being polite!\n");}else{

print ("That was not very polite!\n");}

String – Pattern ExampleString – Pattern Example#!/usr/bin/perlprint ("Enter a variable name:\n");$varname = <STDIN>;chop ($varname);# Try asd$asdas... It gets accepted!if ($varname =~ /\$[A-Za-z][_0-9a-zA-Z]*/) {

print ("$varname is a legal scalar variable\n");}elsif ($varname =~ /@[A-Za-z][_0-9a-zA-Z]*/) {

print ("$varname is a legal array variable\n");}elsif ($varname =~ /[A-Za-z][_0-9a-zA-Z]*/){

print ("$varname is a legal file variable\n");}else{

print ("I don't understand what $varname is.\n");}