1. COMP234-Perl Variables, Literals Context, Operators Command Line Input Regex Program template

2. Variables

3. Variable Names The name of a variable consists of the character $ (or @ or %) followed by at least one letter or underscore, which is followed by any number of letters, digits, or underscore characters. The following are examples of legal variable names: –$x –$_x –$var –$my_variable –$var2 –$a_new_variable

4. Variable Names These, however, are not legal variable names: –variable # the $ character is missing –$ # there must be at least one letter –$47x # second character must be a letter –$variable! # you can't have a ! in a variable name –$new.var # you can't have a. in a variable name

5. Variable Names Variable names should be meaningful –$LineCount – is better than $Count Meaningful names make programs more understandable Therefore easier to debug

6. Good Variable Names Variables answer a question about a thing Names should tell you the what the thing is and what the question is $count –Bad, what is being counted? $RecordCount –Better, but what records? $LogMessageCount –Better still, relates to actual contents of record

7. Variable Names Perl variables are case-sensitive. This means that the following variables are different: –$VAR –$var –$Var Initial caps is a useful convention for long names –$MultiWordName Some prefer Underlines –$Multi_word_name

8. Declaring Variables Perl variables don't have to be pre-declared But if you do, perl can warn you when you spell a name wrong my $variablename Declares $variablename and makes it a private variable This line, put at the start of your program, turns on warnings, and is a good idea. use warnings; You should use this in future programs

9. Literals

10. Types Literals come in types, like variables, but more types Scalars –Numbers –Strings –Boolean and special values Lists (= arrays) Hashes

11. Numbers Stored internally as double precision floating point, or signed integers 12345 # integer 12345.67 # floating point 6.02E23 # scientific notation 0xffff # hexadecimal 0377 # octal 4_294_967_296 # underline for legibility

12. Strings, Single Quote String literals are normally, but not always quoted Single quotes are almost absolute –Except for /' which is interpreted as ' –And // which is interpreted as / 'hello' = hello 'don\'t' = don't '' = the null string (no characters) 'silly\\me' = silly\me

13. Strings, Double Quote Variables and escape sequences are interpolated $Text = 'TEXT' “Some $Text” = SomeTEXT “Some\$Text” = Some$Text “Some\n” = Some, plus newline “Some\”” = Some” “Some\\” = Some\

14. Strings, No Quotes Where it isn't ambiguous, quotes can be omitted –when the string can't be interpreted as something else @days = (Mon,Tue,Wed,Thu,Fri); print STDOUT hello, ' ', world, "\n"; These unquoted strings are called barewords, and are considered a bad thing use warnings; warns about use use strict subs; prohibits barewords use qw( ) makes a list out of barewords qw(foo bar baz) = (“foo”,”bar”,”baz”)

15. Context Behavior of perl operators and values varies according to context Contexts are something like value types: –List Context –Scalar context Numeric String Boolean Void Interpolative

16. Determining Context Often determine by left side of an assignment $Var = qw(a b c) –The scalar variable establishes scalar context so this is the same as $Var = 'c' Evaluating an array in scalar context provides its length $length = @array # gets length of # @array Some operations like print establish a list context

17. Numeric, String, Boolean These scalar sub-contexts sometimes determined by operators or functions Numeric operators == != = Numeric functions like abs() establish numeric context String Operators –eq ne le ge Boolean operators –and && or || not ! xor

18. Precedence of Boolean Operators “and” “or” and “not” have very low precedence compared to && || and ! So this works: open HANDLE, “file” or die “msg” open (HANDLE, “file”) or die “msg” But this doesn't: open HANDLE, “file” || die “msg” open HANDLE, (“file” || die “msg”)

19. Precedence of Boolean Operators On the other hand, this doesn't work $x = $y or $z; means ($x = $y) or $z; But this does: $x = $y || $z; means $x = ($y || $z); Better just to insert your own parenthesis than to trust to precedence

20. Command Line Input

21. Many programs run from the command line take command line parameters –Perldoc -f print –“print” is a parameter passed to the perldoc program on the command line Perl programs can also be passed parameters on the command line perl sum.pl 1 2 Total is 3

22. Using Command Line Input Command line parameters are passed to the perl program as an array called @ARGV $sum = $ARGV[0] + $ARGV[1]; print "Total is $sum\n" ; cmblap:~/samples # perl sum.pl 100 5 Total is 105 cmblap:~/samples #

23. How Many Parameters Sometimes a program wants to know how many parameters are passed This is equivalent to knowing the size of the @ARGV array $size = @ARGV gets the size by evaluating @ARGV in a scalar context scalar (@ARGV) does the same $#ARGV is similar –Returns index of last item scalar(@ARGV) = $#ARGV + 1

24. Regular Expressions

25. Widely used in unix world Very high powered “wild cards” Used to describe complex text patterns And then do things with text patterns – Is this pattern in that text? – Extract the text that matches this pattern – Change the text that matches this pattern

26. Very Basic Regex Any string of characters can be a regular expression that “matches” itself To indicate a string is a regex enclose it in slashes – /abc/ – the string “abc” matches the regex /abc/ – So does the string “abcdefg”

27. Regex operators The eq operator tests a whole string for equality with another ( == for numeric context) The =~ operator tests a string and a regex $a =~ /regex/ – tests to see if the string matches the regex Can leave out the operator – if ( /abc/ ) is a short form for – if ( $_ =~ /abc/ )

28. Multipliers /a*/ means 0 or more a's /a+/ means one or more /a?/ means 0 or 1 Notice that /a+/ is the same as /aa*/

29. General Multipliers /a{5,9}/ matches five to nine a's /a{5,}/ matches five or more a's /a{5}/ matches five a's Note a* same as a{0,} a+ same as a{1,} a? same as a{0,1}

30. Wildcards Dot matches anything /.../ matches any three characters /.*/ matches any string of any length /.*a/ matches any string that contains an a –So does /a/ /a.*a/ matches any string with at least two a's

31. Anchors ^ means the start of a string $ means the end of a string /^a.*/ matches any string that starts with “a” /^a..b$/ – matches four character strings starting with “a” and ending with “b” The ^ and $ don't match a character – they just “anchor” the match to the ends of the string

32. Examples $a = “hello world” $a =~ /^he/ $a =~ /l{2}/ $a =~ /l.$/ $a =~ /^.e.* w.{4}$/

33. What matches What? "hello world" =~ /.*l/ – the “l” matches the last l – because.* is “greedy” "hello world" =~ /.*l.*o/ – now it matches the second “l” "hello world" =~ /.*l.{2} / – now it matches the first

34. Non-greedy Multipliers.*? matches the shortest possible string "hello world" =~ /.*l/ “l” matches the last “l” "hello world" =~ /.*?l/ now it matches the first

35. Regular Expressions Used to describe complex text patterns And then do things with text patterns – Is this pattern in that text? – Extract the text that matches this pattern – Change the text that matches this pattern

36. Extracting Strings Parenthesis -- “(“ and “)” – can be used to group parts of a pattern – as long as what they contain is a sensible pattern Parenthesis do not affect the match – if it matches without them it will still match with them But they let you find out what did match

37. $1 $2 $3 The variables $1 $2 etc contain the string that matched the part in parentheses “hello world” =~ /(.*)l/ – $1 contains: hello wor “hello world” =~ /(.*?)l/ – $1 contains: he Can also use this syntax: ($string) = “hello world” =~ /(.*?)l/ –$string contains he Doesn't work without ( ) –Has to be list context

38. \1 \2 \3 \1 has the same value as $1 but can be used in the regex “hello world” =~ /(.) / – $1 contains an “o” “hello world” =~ /(.) (.*)\1/ – is true – \1 matches “o” in “world” – $2 eq “w”

39. Character Classes Fussier than. [0-9] matches one character from 0123456789 [a-z] matches lower case letters [A-Z] upper case [aeiou] matches a vowel [0-9] same as [0123456789]

40. Abbreviations \d –[0-9] \s –Whitespace \w –[0-9a-zA-Z_] \D –[^0-9] \b matches a boundary between a word character and a non-word character

41. Reading Read about regular expressions in your text Or visit –http://search.cpan.org/~rgarcia/perl- 5.10.0/pod/perlrequick.podhttp://search.cpan.org/~rgarcia/perl- 5.10.0/pod/perlrequick.pod

42. Program Template

43. Programs should start with comment block like this: # Author: # Name: # Version: # Parameters: # Description: # C H A N G E L O G # Date Author Description # Change log could be omitted on ephemeral programs

44. Program Template Create a program template with your name filled in –template.pl Copy it to create new programs copy template.pl lab4.pl Fill in the rest of the comments after copying

45. This week's lab This week's lab uses regular expressions Read: http://search.cpan.org/~rgarcia/perl-5.10.0/pod/perlrequick.pod http://search.cpan.org/~rgarcia/perl-5.10.0/pod/perlrequick.pod Or chapter 12 in Perl for Dummies Or chapter 7 in Learning Perl Or Pattern Matching in chapter 2 of Programming Perl Free perl books in pdf format: http://it-ebooks.info/search/?q=perl&type=title