Perl Basics

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Perl Basics Perl Basics A Perl Tutorial NLP Course - 2003

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Perl Basics. A Perl Tutorial NLP Course - 2003. 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 Scripts. Windows Download ActivePerl from ActiveState - PowerPoint PPT Presentation

Transcript of Perl Basics

Page 1: Perl Basics

Perl BasicsPerl Basics

A Perl Tutorial

NLP Course - 2003

Page 2: Perl Basics

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

Page 3: Perl Basics

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/local/bin/perl

Make the script executable% chmod +x script_name

Run the script% ./script_name

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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

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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

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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

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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

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Operator Operation

||, or logical or

&&, and logical and

!, not logical not

xor logical xor

Logical OperatorsLogical Operators

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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

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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

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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)

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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

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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 [])

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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

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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 {})

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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

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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");

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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" );}

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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

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Pattern Matches Pattern Matches

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

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

/^def/ def word /d.*f/ df, daffff

/^def$/ def /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

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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

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BackreferencesBackreferences Memory of matched portion of input

/[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 (.) in the previous pattern is $1

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Pattern Matching OptionsPattern Matching Options

Escape Sequence

Description

g Match all possible patterns

i Ignore case

m Treat string as multiple lines

o Only evaluate once

s Treat string as single line

x Ignore white-space in pattern

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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"

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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.

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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);

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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");}

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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");}