Perl

Perl notes 2 Perl Perl - Practical extraction report language –for text files –system management –combines C, SED, AWK, SH –interpreted –dynamic

Perl notes 3 Data Structures scalars$num associative arrays%num $num[50] –50th element of the array num $#num –last index of num

Perl notes 4 Examples #! /usr/local/bin/perl -w # find the sum of a list of numbers from STDIN # one number per line $sum = 0; while( ) { $sum += int $_; } print "the sum is $sum\n";

Perl notes 5 Examples #!/usr/bin/perl -w # find the sum of a list of numbers from STDIN # several numbers per line $sum = 0; while( ) = split; foreach { $sum += int $_; } print "the sum is $sum\n";

Perl notes 6 Average #!/usr/bin/perl -w # find the average of a list of # numbers from STDIN # several numbers per line $sum = 0; $count = 0; while( ) = split; foreach { $sum += int $_; $count++; } print "the average is ", $sum/$count, "\n";

Perl notes 7 median #!/usr/bin/perl -w # find the median of a list of number # from STDIN # several numbers per = (); while( ) = split ); = if($#nums % 2) { $median = ($nums[($#nums - 1)/2] + $nums[($#nums + 1)/2])/2; } else { $median = $nums[$#nums/2]; } print "the median is $median\n";

Perl notes 8 Output? #!/usr/bin/perl = ("one", "two", "three"); "\n"; $stuff = ("one", "two", "three"); print $stuff, "\n"; $stuff print $stuff, "\n"; onetwothree8 three 3

Perl notes 9 Pattern Matching m// s/// Modifiers icase-insensitive mmultiple lines ssingle line xextend

Perl notes 10 Regular Expressions Code Meaning \w Alphanumeric Characters \W Non-Alphanumeric Characters \s White Space \S Non-White Space \d Digits \D Non-Digits \b Word Boundary \B Non-Word Boundary \A ^At the Beginning of a String \Z $At the End of a String. Match Any Single Character

Perl notes 11 Regular Expressions * Zero or More Occurrences ? Zero or One Occurrence + One or More Occurrences { N }Exactly N Occurrences { N,M } Between N and M Occurrences.* Greedy Match, up to the last thingy.*? Non-Greedy Match, up to the first thingy [ set_of_things ] Match Any Item in the Set [ ^ set_of_things ]Does Not Match Anything in the Set ( some_expression )Tag an Expression $1..$N Tagged Expressions used in Substitutions

Perl notes 12 Rules Rule 1 –The engine tries to match as far left as it can Rule 2 –The regular expression is regarded as set of alternatives. Tries them left to right. (see page 61) Rule 3 –Items that have choices match from left to right /x*y*/ Rule 4 –Assertions –^ $ \b \B \A \Z \G (?…) (?!…)

Perl notes 13 Rules Rule 5 –A quantified atom matches only if the atom itself matches some number of times allowed by the quantifier Maximalminimal {n,m}{n,m}? {n,}{n,}?At least n {n}{n}?Exactly n **?0 or more ++?1 or more ???0 or 1

Perl notes 14 Rules Rule 6 –Each atom matches according to its type –(…) ==> grouping + storage $1, $2 –. matches any char except \n –[…] groups –Special characters \a \n \r … –\1 \2... backreference to (…) –\033 octal char –\xf7 hex char –\cD control char –any other \ matches the char itself

Perl notes 15 precedence () (?: ) Repetition Sequence | alteration

Perl notes 16 How do you fix it? /(‘[^’]’*’)/

Perl notes 17 Examples s/^([^ ]) +([^ ]+)/$2 $1/ /(\w+)\s*=\s*\1/ /.{40,}/ /^((\d+\.?\d*|\.\d+)$/ if (/Time: (..):(..):(..)/){ $hours = $1; $minutes = $2; $seconds = $3; }

Perl notes 18 Default STDIN sub foo{ my $x = shift; default in the main while($_ = shift) { if(/^-(.*)/){ process_optein($1); } else { process_file($_); }

Perl notes 19 Reading a stream open FIN, “myfile” or die; while ( ){ # do something with $_ } foreach ( ){ # do something with $_ } print sort ;

Perl notes 20 Reading a stream # print a = ; foreach ( 0..$#f ) { if[$[$_] =~ /\bShazam\b/){ $lo = ($_ > 0)? $_ -1 : $_; $hi = ($_ < $#f) )? $_ +1 : $_; print map{“$_: $f[$_]”} $lo.. $hi; }

Perl notes 21 Sorting sort numerically sub numerically { $a $b = sort numerically (16, 1, 8, 2, 4, 32); = sort { $a $b } (16, 1, 8, 2, 4, = sort{uc($a) cmp uc($b)} qw(this is a test); = sort { $b $a } (16, 1, 8, 2, 4, 32);

Perl notes 22 example #! /usr/bin/perl -w # This script will count the frequency of distinct words # in the file that is given as an argument. # Warning: Error checking is minimal! die "usage: $0 file\n" while(<>){ tr/A-Z/a-z/; # translate to = split(/[\W]+/,$_); # split into words foreach $list{$_}++; # increment the counter } foreach $key (sort {$list{$b} $list{$a}} keys %list) { print $key, ' = ', $list{$key}, "\n"; }

Perl notes 23 Tokenizing # tokenize an arithmetic expression while($_){ if(/^(\d+)/) { ‘num’, $1; } elsif(/^([+\-\/*()])/) { ‘punct’, $1; } elsif (/^([\d\D])/) { die “invalid char $1 in input”; } $_ = substr($_, length $1); } substr slows things down –cut start of string

Perl notes 24 Tokenizing 2 while(/ (\d+) | ([+\-\/*()]) | ([\d\D])/gx) { if($1 ne “”){ ‘num’, $1; }elsif ($2 ne “”) { ‘punct’, $2; }else { die “invalid char $3 in input”; }

Perl notes 25 Tokenizing 3 { if(/\G(\d+)/gc) { ‘num’, $1; } elsif(/\G([+\-\/*()])/gc) { ‘punct’, $1; } elsif (/\G([\d\D])/gc) { die “invalid char $1 in input”; }else{ last; } redo; }

Perl notes 26 Use split for clarity ($a, $b, $c) = /^(\S+)\s+(\S+)\s+(\S+)/; ($a, $b, $c) = split /\s+/, $_; ($a, $b, $c) = split; Get the fifth field: ($a) = /[^:]*:[^:]*:[^:]*:[^:]*:([^:]*)/; or ($a) = /(?:[^:]*:){4}([^:]*)/; or ($a) = (split /:/)[4];

Perl notes 27 unpac ps l F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND rt_sig S pts/2 0:00 -tcsh R pts/2 0:00 ps l chomp = `ps l`); ($uid, $pid, $sz, $tt) = unpack A7', $_; print "$uid, $pid, $sz, $tt\n"; }

Perl notes 28 Avoid regex for simple strings do_it() if $answer eq ‘yes’; do_it() if $answer =~ /^yes$/; do_it() if $answer =~ /yes/; do_it() if lc($answer) eq ‘yes’; do_it() if $answer =~ /^yes$/i;

Perl notes 29 #!/usr/bin/perl # remove the comments from a C program $filename = shift or die "usage $0 filename\n"; open FIN, $filename or die "can't open file"; while ( ){ for(split m!("(:?\\\W|.)*?"|/\*|\*/)!){ if($in_comment){ $in_comment = 0 if $_ eq "*/"; } else { if ($_ eq "/*") { $in_comment = 1; print " "; } else { print; } print "\n"; }

Perl notes 30 References $a = ; $scalar_ref = \$a; $array_ref = $hash_ref = \%a; $array_el_ref = \$a[3]; $hash_el_ref = \$a{‘John’};

Perl notes 31 Lists of = ( [“fred”, “barney” ], [“george”, “jane”, “elroy” ], [“homer”, “marge”, “bart” ], ); print $LoL[2][2]; # prints “bart” $ref_to_LoL = [ [“fred”, “barney” ], [“george”, “jane”, “elroy” ], [“homer”, “marge”, “bart” ], ]; print $ref_to_LoL ->[2][2]; Note: $LoL[2][2] implies $LoL[2]->[2]

Perl notes 32 Grow your own = split; ]; }

Perl notes 33 Hashes of Arrays %HoL = ( flinstones => [“fred”, “barney” ], jetsons => [“george”, “jane”, “elroy” ], simpsons => [“homer”, “marge”, “bart” ], ); generation # reading from a file with format: # flistones: fred barney.. while(<>){ next unless s/^(.*?):\s*//; $HoL{$1} = [ split ]; } or while($line = <>){ ($who, $rest) = split /:\s*/, = split ‘ ‘, $rest; $Hol{$who} = ]; }

Perl notes 34 Hashes of Arrays # calling a function for $group (flinstones, jetsons, simpsons) { %HoL($group) = [ get_family($group) ]; ); # append member to existing family $HoL{flinstones} }, “wilma”, “betty”; access $HoL{flinstone}[0] = “fred”;

Perl notes 35 Packages, Modules, and Object Classes