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Perl BasicsPerl Basics
A Perl Tutorial
NLP Course - 2006
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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
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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
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Basic SyntaxBasic Syntax
Statements end with semicolon ; Comments start with #
Only single line comments
Variables You dont 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 thevalue of the string
Check 01_printDate.pl
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String Operation Arithmetic
lt less than
eq equal to ==
le less 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 commaseparated 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 awhite-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 last
print("Numbers (1-9): ",
pop(@numbers_10),
"\n");
# Remove the first
print("Numbers (2-9): ",
shift(@numbers_10),
"\n");
# Combine two ops
print("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$instrument\n" );
}
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Pattern MatchingPattern Matching
A pattern is a sequence of characters to besearched for in a character string /pattern/
Match operators =~: tests whether a pattern is matched !~: tests whether patterns is not matched
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Pattern Matches Pattern Matches
/def/ "define" /d.f/ dif
/\bdef\b/ a defword /d.+f/ dabcf/^def/ defin start of
line
/d.*f/ df, daffff
/^def$/ defline /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 Ranges
EscapeSequence
Pattern 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
Memorize the matched portion of input
Use 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 thepattern is matched \1 in the previous pattern is what is matched by (.)
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Pattern Matching OptionsPattern Matching Options
EscapeSequence
Description
g Match all possible patterns
i Ignore case
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+)/[$1]/
Result: "abc[123]def
Use of backreference!
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Predefined Read-only VariablesPredefined Read-only Variables$& 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 valueselsewhere if you
need 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 alloccurrences of the regular expression within that string. The parts of the stringthat don't match the regular expression are returned in sequence as a list ofvalues.
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 filetry 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");
$question = ;
# what about capital P in "please"?if ($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");$varname = ;chop ($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");}