1. System Administration Introduction to Scripting, Perl Session 5 – Fri 23 Nov 2007
References:
Perl man pages
Albert Lingelbach, Jr.

2. Review
Intro to shell
Intro to perl
Questions from exercises ?

3. Arrays: introduction
Arrays store a series of values that can be accessed by "index"
Index is an integer
starting at 0 or 1; Perl array indexing starts at 0
the upper limit depends on the language or computer
In Perl, array variables begin
Example:
To access an element: $namearray[i]
Example: $namearray [3] = "testing";

4. Arrays: initialization
The contents of an array can be initialized at declaration:
= ("geoff", "foibe", "oscar");
This is equivalent to:
$namearray[0] = "geoff";
$namearray[1] = "foibe";
$namearray[2] = "oscar";

5. Arrays: size
The index of the last element can be found using the expression
$#array
Notice that this is a scalar expression
What is an expression that will return how many elements are in an array ?
What statements will print out every element of an array ?

6. Arrays: practicum Number of elements in an array:
@array in scalar context
Print every element of an array:
for (my $idx = 0; $idx $idx++) {
print "array[$idx] = $array[$idx]\n"; }

7. Arrays: multiple dimensions
Arrays can have more than one dimension:
= ( (" ", " ", " "), (" ", " ", " "), (" ", " ", " ") );
$gameboard [1, 1] = "X";
$gameboard [0, 0] = "O";
$gameboard [1, 2] = "X";
$gameboard [1, 0] = "O";

8. Arrays: miscellaneous
An array can be sorted =
An array can be reversed =
See man -M /usr/perl5/man perldata for more information

9. Hashes: introduction
Hashes store a series of values that can be accessed by "key"
Key is a scalar (string, number) which is unique among the set of keys in the hash
Value is another scalar, associated with the key
Hashes begin with %
Example: my %phonebook;
To access an element: $hash{key}
Example: $phonebook {"mangula"}

10. Hashes: initialization
The contents of a hash can be initialized at declaration:
my %phonebook = (
"albert" => " ",
"mangula" => " " );
This is equivalent to:
my %phonebook;
$phonebook {"albert"} = " ";
$phonebook {"mangula" = " ";

11. Hashes: useful functions
The set of keys in a hash:
= keys %myhash;
The set of values in a hash:
= values %myhash;
Expression to return the number of elements in a hash ?
Code to display the contents of a hash ?

12. Hashes: practicum Number of elements in a hash:
= keys %hash;
my $length
my $length = $#arr + 1;
Display contents of a hash:
= keys %hash;
for (my $idx = 0; $idx $idx++) {
print "hash {$keys [$idx]} = " . "$hash {$keys [$idx]}\n"; }

13. Hashes: miscellaneous
See man -M /usr/perl5/man perldata for more information

14. Regular Expressions: introduction
Regular expressions (sometimes shortened to "regexp"s) are used for matching text to patterns
Recall shell "wildcards": * ?
Excellent for searching, data validation

15. Regular Expressions: statement syntax
"Matching" operator: =~
Pattern markers: / /
Example:
if ($s =~ /$pattern/) {
print "$s matches $pattern\n"; }
else {
print "$s does not match $pattern\n";

16. Regular Expressions: pattern syntax
any letter or number character represents itself
"abc" matches /abc/
| or: one of many sub-patterns
() group sub-patterns
"one" or "two" or "three" match /(one|two|three)/
"one" matches /(one)/, /(on)(e)/
[] group single character options
"a" or "b" or "c" match /[abc]/
"af" or "ag" match /[abc][fgh]/
"ab" does not match /[abc][fgh]/

17. Regular Expression pattern syntax continued
[^ ] not in set
"a" or "b" matches /[^fgh]/
"f" does not match /[^fgh]/
Placement:
^ beginning of string
$ end of string
"abc" matches /abc/, /^abc/, /abc$/, /^abc$/
"xabc" matches /abc/, /abc$/
"xabc" does not match /^abc/ or /^abc$/
"abcx" matches /abc/, /^abc/
"abcx" does not match /abc$/, /^abc$/

18. Regular Expression pattern syntax continued
Quantifiers
( )? zero or one
"caa", "abad" match /ab?a/
"abba" does not match /ab?a/
"acb", "yafoob3" match /a(foo)?c/
"afoofooc" does not match /a(foo)?c/
( )* zero or more
"xaa", "yabbbbbbba" match /ab*a/
( )+ one or more
"aa", "abbbb" do not match /ab+a/

19. Regular Expression pattern syntax continued
More quantifiers
( ){n} exactly n occurrences
"afoofoofoob" matches /a(foo){3}b/
"afoofoob" does not match /a(foo){3}b/
( ){i, j} between i and j occurrences
"afoobarbarb" matches /a(foo){1,3}(bar){2,4}b/
"afoobarbarb" does not match /a(foo){2,3}(bar){2,4}b/
( ){i, } i or more occurrences
"afoob" does not match /a(foo){2,}b/
"afoofoofoofoofoofoob" does match /a(foo){2,}b/

20. Regular Expression pattern syntax continued
\ is "escape" character
use in front of special character to match it literally
"a(b" matches /a\(b/
/a]b/ is not a correct regular expression
use as marker for pre-defined sets/patterns (next slide)‏
. (dot, period) matches any one character
"abb", "a(b" match /a.b/
"abx" does not match /a.b/

21. Regular Expression pattern syntax continued
Pre-defined patterns
\s whitespace (space, tab, newline)‏
\S non-whitespace character (same as [^\s])‏
\d digit
\w word character (a-z, A-Z, 0-9, _)‏

22. Regular Expressions: miscellaneous
There are more pre-defined characters
There are more features
For more information: man -M /usr/perl5/man perlrequick man -M /usr/perl5/man perlretut man -M /usr/perl5/man perlre

23. Review
= ("zero", "one", "two"); $array[idx]; $#array;
Hashes: %hash = ("a" => 1, "b" => 2); $hash {$key}; keys %hash; values %hash;
Regular expressions: =~ // . ? + * () [] [^] ^ $ {,}

24. Next up: Process management No class until January
See for exercises
any questions