Perl tutorial

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Perl tutorial
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Running Perl code

• from command line
• perl e print 12/4,\n
• from a file
• perl myprogram.pl
• interactively
• gdsh

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Variables

• A scalar represents a single value
• room 219
• name Ari
• sentence name sits in room room

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Data structures in Perl

• An array represents a list of values
• _at_data (Ari, 219, Maa-123.490)
• data0, data,_at_data2..3,_at_data
• A hash represents a set of key/value pairs
• data (namegtAri, roomgt219,coursegtMaa-123.4
  90)
• _at_k keys data

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References

• References are the key to complex data structures
• people-gtAri-gtroom-gtsize
• people is a reference to a hash
• people
• Ari gt
• room gt
• size gt 12

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Conditional constructs

• if ( condition )
• elsif ( other condition )
• Negated version of if is unless

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Looping constructs

• while ( condition )
• Negated version of while is until

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Looping constructs (for)

• for ( i0 iltmax i )
• for ( _at_array )
• for my key ( keys array )

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Builtin operators and functions

• Arithmetics, boolean logic, ...
• _at_sorted sort _at_data
• _at_sorted sort altgtb _at_data
• _at_sorted sort a cmp b _at_data

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File I/O

• open(INFILE, "input.txt) or die "Cant open
  input.txt !"
• open(OUTFILE, "gtoutput.txt") or die "Cant open
  output.txt !"
• open(LOGFILE, "gtgtmy.log") or die "Cant open
  logfile !"
• my line ltINFILEgt
• my _at_lines ltINFILEgt
• while (ltINFILEgt) assigns each line in
  turn to _
• print "Just read in this line _"
• close INFILE

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Regular expressions
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What are regular expressions?

• at command prompt cp .html
• in web search garden?
• , ? are wildcards
• search with different criteria
• search and replace
• splitting input
• concise, cryptic looking code...
• s/(A_Z1)(a-z)\.sgml/\1\2\.html/g

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What can I do with regexes?

• look out for interesting things in input stream
• input stream text files on your computer, web
  pages, data produced by a program, ...
• you do not need to know the exact format of the
  input stream
• often line by line, but not necessarily
• text
• for example sort spam

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Drawbacks and alternatives

• regexes are easier to write that read
• maintenance is a problem
• alternatives
• text editor / word processor macros
• spreadsheet import
• ordinary code
• more readable, easier to use
• regexes flexibility, portability, conciseness,
  fault-tolerance, programmability

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How do I use regexes?

• regexes are not a programming language but an
  extension in many
• Perl, Tcl, Python, C, Java, Visual Basic
• sed and grep unix tools
• generally similar but not all implementations are
  alike!
• in these slides I use Perl unless stated otherwise

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Why regular expressions?

• Stephen Kleene (b. 1909, ? 1994)
• the algebra of regular sets
• Kleene star

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Howto (1)

• In normal text string geo is one instance.
• In a regex the string geo is a template that
  matches all instances of characters g, e, and
  o all in a row
• grep geo mytextfile gthits
• ? all lines in mytextfile which contain word
  geo are stored in a new file hits

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Howto (2)

• sed s/geo/ego/ mytextfile gthits
• ?all lines in mytextfile are copied to hits but
  lines containing string geo are changed so that
  geo becomes ego
• you may want to use modifier /g to change all
  instances of geo to ego

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More complicated patterns

• alphanumeric characters match themselves
• character classes
• \w \W \s \S \d \D ... ...
• metacharacters and anchors
• .
• quantifiers
• ? num num, min,max
• grouping and alternation
• ()

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More patterns

• Metacharacters
• \t \n \r
• Word boundary (zero width)
• \b \w\W or \W\w
• Escape, I.e. how to match . for example
• \. \

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Capturing

• ( ... ) creates capture buffers
• to refer to the nth buffer use
• \n within the match
• n outside the match
• Examples
• s/( ) ( )/2 1/
• if (/(.)\1/) print 1 is now\n
• if (/Time (..)(..)(..)/) print it is
  12.3 now\n
• match variables are dynamically scoped until the
  end of the enclosing block or next successful
  match

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Modifiers

• Case insensitive matching
• /i
• Global, I.e. matching all occurrences
• /g
• Evaluate code in regex
• /e

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• For a regex to match the entire regex must match

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Greediness

• By default, a quantified subpattern is greedy,
  that is, it will match as many times as possible
  (given a particular starting location) while
  still allowing the rest of the pattern to match.
• To match a minimum number of times, follow the
  quantifier with a ?.
• For example vs. ?

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Example of greediness

• _ "I have 2 numbers 53147"
• if ( /(.)(\d)/ )
• print "Beginning is lt1gt, number is lt2gt.\n"
• Output Beginning is ltI have 2 numbers 53147gt,
  number is ltgt.
• (.)(\d) ltI have 2 numbers 5314gt lt7gt
• (.?)(\d) ltgt ltgt
• (.?)(\d) ltI have gt lt2gt
• (.)(\d) ltI have 2 numbers 5314gt lt7gt
• (.?)(\d) ltI have 2 numbers gt lt53147gt
• (.)\b(\d) ltI have 2 numbers gt lt53147gt
• (.\D)(\d) ltI have 2 numbers gt lt53147gt

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• A regular expression is merely a set of
  assertions that gives a definition of success.
• Warning it is possible to devise regexes which
  take exponential time to solve.
• Sometimes it is best to use several (simple)
  regexes instead of one (complicated)

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Perlishnesses (1)

• In if (/(\d)/) ... Perl tries to match the
  contents of the default variable (_) against the
  regex
• Sometimes the default variable gets the value you
  want.
• Sometimes you need to set it explicitly.

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Perlishnesses (2)

• The split /regex/, string splits string into a
  list of substrings and returns that list
• x "1.618,2.718, 3.142"
• _at_const split /,\s/, x
• const0 '1.618'
• const1 '2.718'
• const2 '3.142'