Introduction to Perl Scott Hazelhurst - - PowerPoint PPT Presentation

Introduction to Perl

Introduction to Perl

Scott Hazelhurst http://www.bioinf.wits.ac.za/~scott/perl.pdf August 2013

Introduction to Perl Introduction and Motivation

Introduction and Motivation

Practical Extraction and Report Language

◮ General language – Intended for systems

programming, scripting

◮ Popular with systems programmers & Web

developers

◮ Relatively new language (ca. 1990) ◮ Big language – support for concurrency and OO. ◮ Portable.

Introduction to Perl Introduction and Motivation

Perl powerful and flexible language:

◮ Many devotees ◮ Many criticisms of the language

Easy to write difficult-to-understand code.

◮ Focus on writing readable code (for yourself,

• thers)

Introduction to Perl Introduction and Motivation

Objectives

◮ Describe basic features of Perl ◮ Write simple Perl programs ◮ Use basic matching facilities

Lots of resources:

◮ Books ◮ http://cpan.mirror.ac.za/,

www.cpan.org, www.perl.org

◮ perldoc, info perl

Assume: knowledge of programming, C-like language

Introduction to Perl Perl’s Data Structures

Perl’s Data Structures I

Perl is an imperative language.

◮ State is represented by a set of variables. ◮ Program is an ordered sequence of commands. ◮ Computation is accomplished by the execution

• f these commands in the specified order

Will explore Perl’s OO features later.

Introduction to Perl Perl’s Data Structures

Flexible (perhaps too flexible) language.

◮ Scalars: numbers, strings ◮ Arrays, lists ◮ Hash tables ◮ References

Variables have a special prefatory character to indicate to which genre of variable (%, $, @, . . . )

Introduction to Perl Perl’s Data Structures

Feature Warning

◮ Variables automatically declared ◮ Variables given default values ◮ Implicit type coercion common ◮ Meaning dependent on context

Good idea to use the “ use strict; use warnings” pragmas

Introduction to Perl Perl’s Data Structures Scalar types

Scalar types

All scalar variables have a $ as prefix: e.g. $x, $year$, $etc. $c= 10; $f = 9/5* $c +32; $cname = ’Introduction to Perl ’; print "Course $cname: size $c\n";

Introduction to Perl Perl’s Data Structures String operations

Interpolation

◮ Inside a double-quoted string, Perl does variable

interpolation, and interprets escape characters print "Course $cname size \t $f\n;";

Introduction to Perl Perl’s Data Structures String operations

Interpolation

◮ Inside a double-quoted string, Perl does variable

interpolation, and interprets escape characters print "Course $cname size \t $f\n;";

◮ Not done inside single-quoted strings.

print ’Course $cname size \t $f\n’;

Introduction to Perl Perl’s Data Structures String operations

Other string operations

Concatenation: . $fruits= ’apples and pears ’; $base = ’pies ’; $full = $fruits . ’ make ’ . $base; $full .= ’: ’ . fruits;

◮ Length of string: length. Substrings: substr ◮ Repeat operator: x ◮ Powerful & flexible string matching and processing –

regular expressions.

◮ Type conversion to and from integers as necessary !!?!!

Introduction to Perl Perl’s Data Structures Artithmetic operations

Arithmetic Operations

The basic Perl arithmetic operations are: +, -, *, /, %, **.

◮ Default: floating point arithmetic ◮ There are many arithmetic procedures: int,

sqrt,. . . Short-cuts: Perl has many C-like features. eg. $x = 3; $x += $y+1; $x++;

• -$y;

Introduction to Perl Perl’s Data Structures Truth and Logical operations

Truth and Logical operations

• 1. Any string is true except for the empty string

and ”0”;

• 2. Any number is true except for 0
• 3. Any reference is true
• 4. Any undefined value is false

Introduction to Perl Perl’s Data Structures Truth and Logical operations

Logical operators

High binding Low binding && and ||

• r

! not Short-circuit evaluation! Logical expressions returns the last value evaluated. e.g. 2 or 3 and 5 2 and (0 or 5) $a ||= 2

Introduction to Perl Perl’s Data Structures Truth and Logical operations

Relational operators

Comparison Numeric String Equal, Not equal ==, != eq, ne Less than (equal) <, <= lt, le Greater than (equal) >, >= gt, ge Comparison <=> cmp Smart matching ~~ ~~

Introduction to Perl Perl’s Data Structures Truth and Logical operations

File operators

Example Name Example Name

• e $a

File Exists

• T $a

File is a text file

• r $a

Readable file

• w $a

Writable file

• d $a

File is a directory

• f $a

Regular file

Introduction to Perl Standard input and output

Standard input and output

Default: file handle STDIN associated with keyboard (input), STDOUT with console output.

◮ Input is line oriented.

Introduction to Perl Standard input and output

Output print command by default sends output to STDOUT. print "Hello"; same as print STDOUT "Hello";

Introduction to Perl Standard input and output

Input A reference to <STDIN> waits for input from console.

◮ data typed in is returned

print "Enter the temp in C: "; $c = <STDIN>; print "Temperature in F is ". (9/5*$c+32);

Introduction to Perl Control Structures Making decisions

Control structures

Conditional

◮ if/else/elsif:

if ($x < $y) {$x=1} else {$y=1};

◮ unless: unless ($ok) { die "Error in file I/O"; } ◮ no explicit switch/case ◮ implicit through use of short-circuit

(-e "f.dat") or ($numf++)

◮ ternary conditional operator —

cond ? ex1 : ex2

Introduction to Perl Control Structures Making decisions

Example unless (-e "data.dat") { die "File data.dat not found" };

Introduction to Perl Control Structures Making decisions

Example $c = $a cmp $b; if ($c ==0) { print "The strings are the same .\n } elsif ($c <0) { print "$a comes before $b\n"; } else { print "$b comes before $a\n"; }

Introduction to Perl Control Structures Loops

Loops

◮ while: while (cond} { ... } ◮ for statement: C-style:

for (init; condition; change) { ... }

◮ foreach

Introduction to Perl Control Structures Loops

Example while loop

Read in a list of numbers terminated by 0. Compute the sum. $sum =0; $num=<STDIN >; while ($num != 0) { $sum=$sum+$num; $num=<STDIN >; } print "Sum is $sum\n";

Introduction to Perl Control Structures Loops

use strict; my $i; my $j; for ($i=1; $i <11; $i++) { for ($j=0; $j <$i; $j++) { print "*"; } print "\n"; }

Introduction to Perl Control Structures Loops

use strict; my $i; for ($i=1; $i <11; $i++) { print "*" x $i; print "\n"; }

Introduction to Perl Control Structures Other flow of control. . .

Other flow of control. . .

◮ next, last continue and break ◮ scalar range operator: ..

don’t use unless you’re an expert.

◮ goto

don’t use

Introduction to Perl Simple process control

Simple process control

Can execute Unix commands or executables using the backtick operator ‘ $today = ‘date "+%C%y%m%d"‘ ; print "Today is $today\n";

Introduction to Perl Simple process control

Other system calls

system Similar to backtick but returns return code $x = system("ls"); print "Returned <$x >\n"; exec Similar to system, but does not wait for completion.

Introduction to Perl File operations

File operations

To read data from a file or write it to a file, use a file handle.

◮ Need to associate file handle with external file. ◮ File can be virtual – could be a pipe.

Introduction to Perl File operations Files in general

Files in General

File handle is the Perl construct used to manipulate files

◮ Open it – associate with external file or pipe ◮ Use it ◮ Close it

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

◮ open(DATAF, ">figs.dat")

write

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

◮ open(DATAF, ">figs.dat")

write

◮ open(DATAF, ">>figs.dat")

append

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

◮ open(DATAF, ">figs.dat")

write

◮ open(DATAF, ">>figs.dat")

append

◮ open(DATAF, "| output-pipe-cmd");

set up output filter

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

◮ open(DATAF, ">figs.dat")

write

◮ open(DATAF, ">>figs.dat")

append

◮ open(DATAF, "| output-pipe-cmd");

set up output filter

◮ open(DATAF, "input-pipe-cmd| "); set up

input filter

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

◮ open(DATAF, ">figs.dat")

write

◮ open(DATAF, ">>figs.dat")

append

◮ open(DATAF, "| output-pipe-cmd");

set up output filter

◮ open(DATAF, "input-pipe-cmd| "); set up

input filter

Introduction to Perl File operations Files in general

◮ open(DATAF, "figs.dat")

read

◮ open(DATAF, "<figs.dat")

read

◮ open(DATAF, ">figs.dat")

write

◮ open(DATAF, ">>figs.dat")

append

◮ open(DATAF, "| output-pipe-cmd");

set up output filter

◮ open(DATAF, "input-pipe-cmd| "); set up

input filter To print to a file that is open for writing, use the related file handle in the print statement.

Introduction to Perl File operations Input from files

Input from files

Suppose that INP is a file handle. A reference to <INP> does the following:

◮ Returns the next line of input; ◮ Consumes the input – in the case of a file,

advances the file pointer to the next line of the file.

◮ NB: end-of-line marker read in.

Introduction to Perl File operations Input from files

for($i=0; $i <3; $i++) { $x=<INP >; print "**$x##" } Assuming that file contains apple, banana, cherry.

Introduction to Perl File operations Input from files

for($i=0; $i <3; $i++) { $x=<INP >; print "**$x##" } Assuming that file contains apple, banana, cherry. **apple ##**banana ##**cherry ##

Introduction to Perl File operations Input from files

To add up the numbers in a file with three numbers.

• pen(INP ,"nums.txt");

$x1 = <INP >; $x2 = <INP >; $x3 = <INP >; print "Answer is " . $x1 + $x2 + $x3;

• r
• pen(INP ,"nums.txt");

print "Answer is " . (<INP > + <INP > + <INP >);

Introduction to Perl File operations Input from files

What’s the difference between

print "Answer is " . (<INP > + <INP > + <INP >); and print "Answer is . (<INP > + <INP > + <INP >) ";

Introduction to Perl File operations Input from files

Add up odd numbers in file

print "Enter the file name: "; $namef = <STDIN >;

• pen(DATAF , $namef );

while($x = <DATAF >) { if ($x % 2) { $sum = $sum + $x; } } close(DATAF ); print "The sum is $sum\n";

Introduction to Perl File operations Input from files

A common Perl idiom is

• pen(DATAF ,$namef) or die "Can’t open

Introduction to Perl File operations Implicit operands

Implicit Operands

Perl uses implicit operands extensively.

◮ We are told that this is a feature. ◮ The main culprit: $_ or $ARG

Set (among other places) by reference to a file

• handle. Many operators use $ARG if not given

explicit operator explicitly.

◮ The following prints out the contents of a file:

while (<DATAF>) {print};

Introduction to Perl File operations Implicit operands

Exercise

Write a Perl program that reads integers from a file called nums.dat and counts how many numbers are

◮ less than zero ◮ between zero and 10 ◮ greater than or equal to 11

If there is no such file, your program should print an error message and halt.

Introduction to Perl Arrays/lists

Arrays/lists

Array variables are prefixed by @ my @days; @days = ("Sun", "Mon", "Tue", "Wed", "Thu", "F

◮ Arrays indexed from 0

Introduction to Perl Arrays/lists

Arrays/lists

Array variables are prefixed by @ my @days; @days = ("Sun", "Mon", "Tue", "Wed", "Thu", "F

◮ Arrays indexed from 0 ◮ Individual element are scalars: so $days[0]

Introduction to Perl Arrays/lists

Common to use foreach

@days = ("Sun", "Mon", "Tue", "Wed", " foreach $d (@days) { print "Day $d\n"; }

Introduction to Perl Arrays/lists

Quote words

Short hand for arrays of words @days = qw(Sun Mon Tue Wed Thu Fri Sat

Introduction to Perl Arrays/lists

@ARGV – program’s arguments

@ARGV: Predefined variable array

◮ Contains the program’s arguments – values

passed by the caller.

◮ If the program is run as follows:

./example.pl apple pear 1 2 3 array @ARGV contains those values

◮ $ARGV[0] is apple; ◮ $ARGV[3] is 2;

Introduction to Perl Arrays/lists

Array semantics

Expressions are evaluated in scalar or list context.

◮ Similar or identical expressions can evaluate to

different things in different contexts.

Introduction to Perl Arrays/lists

Array semantics

Expressions are evaluated in scalar or list context.

◮ Similar or identical expressions can evaluate to

different things in different contexts. Context determined by operation:

◮ LHS of assignment determines context ◮ Operators or functions may determine context

Introduction to Perl Arrays/lists

Array context

◮ print @days yields SunMonTueWedThuFriSat ◮ @weekdays = @days[1..5] array $weekdays set to

Mon .. Fri

◮ @days[1] is a one element array

Introduction to Perl Arrays/lists

Array context

◮ print @days yields SunMonTueWedThuFriSat ◮ @weekdays = @days[1..5] array $weekdays set to

Mon .. Fri

◮ @days[1] is a one element array

Scalar context

◮ $numdays = @days; sets $numdays to 7

Introduction to Perl Arrays/lists

Array context

◮ print @days yields SunMonTueWedThuFriSat ◮ @weekdays = @days[1..5] array $weekdays set to

Mon .. Fri

◮ @days[1] is a one element array

Scalar context

◮ $numdays = @days; sets $numdays to 7 ◮

$y = ("Sun","Mon","Tue","Wed","Thu","Fri","Sat") sets $y to Sat

Introduction to Perl Arrays/lists

Array context

◮ print @days yields SunMonTueWedThuFriSat ◮ @weekdays = @days[1..5] array $weekdays set to

Mon .. Fri

◮ @days[1] is a one element array

Scalar context

◮ $numdays = @days; sets $numdays to 7 ◮

$y = ("Sun","Mon","Tue","Wed","Thu","Fri","Sat") sets $y to Sat

◮ $weekdays = $days[1..5] sets $weekdays to 5 ◮ $#days is 6 – the highest index of @days

Introduction to Perl Arrays/lists

Useful functions

◮ push : adds something to the right of the array ◮ pop : removes the rightmost element of the

array.

◮ shift/unshift ◮ sort: returns a sorted list

by default string order ascending, but can be changed.

Introduction to Perl Arrays/lists

Arrays are inherently 1D

◮ Need references to handle multi-dimensional

arrays

Introduction to Perl Arrays/lists Asides

Asides on strings

◮ splitting

@x = split ";", "1;2;3"; foreach $s (@x) { print "$s\n"; }

◮ chop, chomp. ◮ see regexes later

Introduction to Perl Hash tables

Hash tables

Unordered set of scalars which allows fast information retrieval:

◮ Elements accessed/indexed by a string value

associated with it

◮ Variables prefixed by a %, e.g. %currency ◮ Individual elements are scalar:

$currency["South Africa"]

Introduction to Perl Hash tables

$currency["South Africa"]="ZAR"; $currency["Britain"] = "GBP";

Introduction to Perl Hash tables

Example

%day2num = ("Sun",0, "Mon",1, "Tue",2, "Wed",3, "Thu",4, "Fri",5, "Sat",6) Better is %day2num= ("Sun"=>0, "Mon"=>1, "Tue"=>2, "Wed"=>3, "Thu"=>4, "Fri"=>5, "Sat"=>6) Referred to as $day2num{"Wed"}

Introduction to Perl Hash tables

Useful hash operations

◮ keys returns list of keys in hash table.

Order non-deterministic.

Introduction to Perl Hash tables

Useful hash operations

◮ keys returns list of keys in hash table.

Order non-deterministic.

◮ values returns list values stored in the hash

• table. Order of no significance.

Introduction to Perl Hash tables

Useful hash operations

◮ keys returns list of keys in hash table.

Order non-deterministic.

◮ values returns list values stored in the hash

• table. Order of no significance.

◮ sort keys %table lists the keys in

lexicographic order

Introduction to Perl Hash tables

Example

# read in from file while ($name = <INP>) { $reg = <INP>; $carreg{$name} = $reg; } # print out in order of name foreach $n (sort keys %carreg) { print "Car reg of $n is $carreg{$n} \n"; }

Introduction to Perl Hash tables

Can also tell sort how to sort:

◮ sort {$a <=> $b} list sorts the list in

numeric order.

◮

sort {$table{$a} cmp $table{$b}} keys %table sorts the keys so that corresponding hash table elements are in order

Introduction to Perl Hash tables

#print out in order of car reg foreach $n (sort {$carreg{$a} cmp $carreg{$b}} keys %carreg) { print "$carreg{$n} owned by $n\n"; }

Introduction to Perl Procedures

Procedures

Perl has procedures but its parameter passing mechanism is poor.

◮ Suppose we have a procedure plus that adds

up two numbers. Called: $x = &plus($a, $b); where $a and $b are the parameters. Call by value semantics

Introduction to Perl Procedures

Declaring a subroutine

To declare a subroutine: sub NAME BLOCK e.g. sub printhead { print "Name Age Number Balance\n"; } ... &printhead();

Introduction to Perl Procedures

Parameters

Procedures have one formal parameter: @_

◮ The values of the actual parameter are given to

the formal parameter (call-by-value).

◮ @_: a list local to the procedure.

sub plus { sub plus { $x = $_[0]; my ($x,$y) = @_; $y = $_[1]; return $x+$y; return $x + $y; } }

Introduction to Perl Procedures

A common idiom uses shift sub plus { $x = shift @_; $y = shift @_; return $x + $y; }

Introduction to Perl Procedures

Example

A procedure to add up a list: sub addlist {$sum = 0; foreach $num (@_) {$sum += $num}; return $sum;} sub proclist { my ($f1,$f2,@nums) = @_; foreach $n (@nums) {$sum = $sum+$n; } return ($f1*$f2*$sum); } $x = &addlist(1,2,3,5,6,9); $y = &proclist(3,4,1,1,0,2);

Introduction to Perl Procedures

The following does not work (s sub dotprod { (@a,@b) = @_; ... ... } @x = (1,2,3); @y = (4,5,6); &dotprod(@x,@y);

Introduction to Perl Procedures

◮ Formal parameter is an unstructured list (possible danger

in passing multiple lists as parameters)

◮ Forward declarations: sub addlist;

Telling Perl that addlist is a procedure which will be declared elsewhere.

◮ Can have anonymous procedures that get assigned to

variables.

◮ Weakness in parameter system can be overcome by using

references.

◮ Default: all variables are global wherever defined. This is

even recognised by the Perl community to be a Bad Thing Declare variables local inside procedures. Best way: prefix first use of variable in a procedure with with my. Lexical scoping.

Introduction to Perl References

References

References are mechanisms for a variable to refer to something, e.g. (1) another variable; (2) a piece of data; (3); a function Symbolic references Can turn a string into a variable $x = 123; $y = "x"; $$y = 5; print $x;

Introduction to Perl References

Hard references

Can use the \ operator to dereference a variable, and -> to dereference. @a = (10,20,30,40,50); $x = \@a; for $e ( @{$x} ) { print "$e\n"; } print $x->[0];

Introduction to Perl References

Call by reference

sub count { my ($fname , $rcount) = @_; chomp $fname; unless (-T $fname) { return; }

• pen(FINP ,$fname );

while (<FINP >) { $$rcount ++ } close(FINP ); }

Introduction to Perl References

@files = ‘ls -1 $ARGV [0]‘; $n = 0; for $f (@files) { &count($f ,\$n); } print "Total number of lines is $n\n";

Introduction to Perl References

Passing multiple arrays

sub dotprod { ($a,$b)=@_; .... } @x = (1,2,3); @y = (4,5,6); &dotprod(\@x,\@y); Note that only scalars are passed.

Introduction to Perl References

Refs to anonymous arrays and hashes

Square brackets creates an array – returns a reference $a = [ 10, 20, 30, 40];

◮ Error: @a = [ 10, 20, 30, 40];

Curly braces creates a hash – returns a reference $day = {"sun"=>0, "mon"=>1, "tues"=> .....}

Introduction to Perl Modules

Modules

A module is a collection of code, data structures that can be used as a library

◮ Often see it in OO programming

Magic word is use use IO;

◮ To access something is in a module use ::. So,

module::thing

◮ Modules can be nested.

Introduction to Perl Modules

Example

use IO:: Compress :: Bzip2; IO:: Compress :: Bzip2 :: bzip2 ("do.pl", "do.pl.bz

Introduction to Perl Modules

Example

use IO:: Compress :: Bzip2; IO:: Compress :: Bzip2 :: bzip2 ("do.pl", "do.pl.bz Can also import specific things use IO:: Compress :: Bzip2 qw(bzip2 $Bzip2Error ); unless (bzip2 ("do.plx", "do.pl.bz2")) { warn "Compression failed returning : $Bzip2 }

Introduction to Perl Objects

Objects

Data structures which

◮ contain data, know functions that can apply to

them;

◮ anonymous ◮ accessed through references ◮ typically organised in classes

Introduction to Perl Objects

use Bio::Seq; $seqio = Bio::SeqIO ->new(’-format ’ => ’embl ’ , -file $seqobj = $seqio ->next_seq (); $seqstr = $seqobj ->seq (); $seqstr = $seqobj ->subseq (10 ,50); @features = $seqobj -> get_SeqFeatures (); foreach my $feat ( @features ) { print "Feature ",$feat ->primary_tag , " starts ",$feat ->start , " ends ", $feat ->end ," strand ",$feat ->strand ,"\n"; }

Introduction to Perl Regular expressions

Regular expressions, matching, and more

Perl’s regular expression support powerful

◮ concise way of describing a set of strings.

Typical: a command uses a regular expression to process some argument.

◮ Example:

split uses a regex to split a string

$line = "Gauteng;Johannesburg GP 7"; @info = split("[ ;]", $line); ($prov, $capital,$reg, $pop) = split("[ ;] ", $line);

Introduction to Perl Regular expressions

Specifying Regular Expressions

◮ Most characters stand for themselves: F Fred 6312

Introduction to Perl Regular expressions

Specifying Regular Expressions

◮ Most characters stand for themselves: F Fred 6312 ◮ \ | ( ) [ { ^ $ * + ? .

are metacharacters (have special meaning)

◮ escape with a backslash for the chars:

Fred stands for the string (Fred)

Introduction to Perl Regular expressions

Specifying Regular Expressions

◮ Most characters stand for themselves: F Fred 6312 ◮ \ | ( ) [ { ^ $ * + ? .

are metacharacters (have special meaning)

◮ escape with a backslash for the chars:

Fred stands for the string (Fred)

◮ To group things together, use parentheses.

Introduction to Perl Regular expressions

Specifying Regular Expressions

◮ Most characters stand for themselves: F Fred 6312 ◮ \ | ( ) [ { ^ $ * + ? .

are metacharacters (have special meaning)

◮ escape with a backslash for the chars:

Fred stands for the string (Fred)

◮ To group things together, use parentheses. ◮ To specify alternatives, use |

(green|red) apples stands for green apples or red apples

Introduction to Perl Regular expressions

Specifying Regular Expressions

◮ Most characters stand for themselves: F Fred 6312 ◮ \ | ( ) [ { ^ $ * + ? .

are metacharacters (have special meaning)

◮ escape with a backslash for the chars:

Fred stands for the string (Fred)

◮ To group things together, use parentheses. ◮ To specify alternatives, use |

(green|red) apples stands for green apples or red apples

Introduction to Perl Regular expressions

Specifying Regular Expressions

◮ A list of characters in square brackets matches any of the

characters.

◮ [YyNn] matches any of an upper or lower case “y” or

“n”.

◮ [A-Za-z0-9] is all the alphanumeric characters

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace;

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace; ◮ \d digit; \D non-digit;

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace; ◮ \d digit; \D non-digit; ◮ \w a word charater, \W a non-word character

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace; ◮ \d digit; \D non-digit; ◮ \w a word charater, \W a non-word character ◮ . anything but a \n

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace; ◮ \d digit; \D non-digit; ◮ \w a word charater, \W a non-word character ◮ . anything but a \n ◮ m{3} stands for mmm

(map){2,3} stands for mapmap or mapmapmap

◮ m* stands for 0 or more m’s

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace; ◮ \d digit; \D non-digit; ◮ \w a word charater, \W a non-word character ◮ . anything but a \n ◮ m{3} stands for mmm

(map){2,3} stands for mapmap or mapmapmap

◮ m* stands for 0 or more m’s ◮ m+ stands for 1 or more m’s

Introduction to Perl Regular expressions

◮ \n new line; \t tab; \s a whitespace; ◮ \d digit; \D non-digit; ◮ \w a word charater, \W a non-word character ◮ . anything but a \n ◮ m{3} stands for mmm

(map){2,3} stands for mapmap or mapmapmap

◮ m* stands for 0 or more m’s ◮ m+ stands for 1 or more m’s ◮ Many others. Rules long and complex.

Introduction to Perl Regular expressions

Processing with regular expressions

Various commands that allow

◮ finding a pattern matching a regular expression in a string; ◮ extracting out the regular expression; ◮ substituting; ◮ or other modification

e.g. matching, substitution, translation, substitution.

Introduction to Perl Regular expressions

Splitting up input

split(pat,string) Takes the input string and splits the string wherever the pattern pat occurs.

Introduction to Perl Regular expressions

Splitting up input

split(pat,string) Takes the input string and splits the string wherever the pattern pat occurs.

◮ returns a list of strings as a result ◮ can choose whether the split pattern is part of the

returned string or not

Introduction to Perl Regular expressions

Splitting up input

split(pat,string) Takes the input string and splits the string wherever the pattern pat occurs.

◮ returns a list of strings as a result ◮ can choose whether the split pattern is part of the

returned string or not $x = split /\*|::/, $c processes the string $c, splits it wherever a * or :: appear, and returns the split list. So, if $c="Dat: 23 * Mon: 11; LgA -632 ::: LgB -217* a", $x= ?

Introduction to Perl Regular expressions

Matching

◮ m/regexpr/ returns true if the regular expression matches

$_.

Introduction to Perl Regular expressions

Matching

◮ m/regexpr/ returns true if the regular expression matches

$_. The m is optional in most cases (unless slash in string) m-/apple-

Introduction to Perl Regular expressions

Matching

◮ m/regexpr/ returns true if the regular expression matches

$_. The m is optional in most cases (unless slash in string) m-/apple-

◮ To match a particular string use the binding operator: =~

$inp =~ m/(Pascal)|(\WC\W)|(C\+\+)/;

Introduction to Perl Regular expressions

Matching

◮ m/regexpr/ returns true if the regular expression matches

$_. The m is optional in most cases (unless slash in string) m-/apple-

◮ To match a particular string use the binding operator: =~

$inp =~ m/(Pascal)|(\WC\W)|(C\+\+)/;

◮ A number of modifiers available: i (case insensitive) g

(global) Count number of matches in a string: while ($inp =~ m/(Pascal)|(\WC\W)|(C\+\+)/g) { $wrd++ }

Introduction to Perl Regular expressions

Extracting patterns

* Can refer to and extract out subexpressions matched $1, $2, . . .

◮ To find a repeat m/\b(\w+)\s+\1/ ◮ To extract out

while ($line = <INP>) { $line =~ m/(\w+)\s+(\w+)/; $name = $1; $reg = $2; $carreg{$name}=$reg; } Look for repeats:

Introduction to Perl Regular expressions

Substitution

s/regexprA/regexprB/ substitutes regular expression A by regular expression B. Default string processed is $ . s/Apple/Orange/; Replaces the first occurrence of Apple in $ with Orange Similar modifiers to m: i, g,... Use the binder operator to choose the string chosen: $name = "Alan Turing"; $name =~ s/(\w)\w*/\1\./; $c = ($name =~ s/(\w)\w*/\1\./); $c = ($name =~ s/(\w)\w*/\1\./g);

Introduction to Perl Regular expressions

Translation

Use the tr or y operator.

◮ Operates on strings, rather than regular expressions.

tr/range1/range2/ replaces any character in range1 and replaces it with the corresponding character in range2.

◮ $c =~ tr/a-z/A-Z/ change every character in $c to

uppercase.

◮ $cnt = $c =~ tr/a-z/a-z/ sets $cnt to the number of

lower case letters in $c.

Introduction to Perl And more

And more

◮ maths functions ◮ more sophisticated I/O ◮ lots of built in variables: e.g. \$NR ◮ modules, OO ◮ IPC, networking ◮ process control