Introduction to Programming the WWW I CMSC Winter 2004 Lecture 13

2 Today’s Topics Regular expressions (cont’d) CGI scripting discussion

3 Midterm Results Total points of the paper: 50 Highest grade: 47.5 Avg. grade: submitted papers  9 papers with grade >= 40

4 Review: Patterns in String Variables Many programming problems require matching, changing, or manipulating patterns in string variables.  An important use is verifying input fields of a form helps provide security against accidental or malicious attacks. For example, if expecting a form field to provide a telephone number as input, your program needs a way to verify that the input comprises a string of seven digits.

5 Review: The Match Operator The match operator is used to test if a pattern appears in a string.  It is used with the binding operator (“ =~ ”) to see whether a variable contains a particular pattern.

6 Review: Using Character Class Matching any one in a set of characters enclosed within square brackets  foo[bc]ar will match foobar and foocar Ranges can be expressed inside of a character class by using a dash between two characters  [a-g] is equal to [abcdefg]  [0-9] is equal to any digit  [a-zA-Z] Negative character class: use the caret (^) symbol as the first thing in the character class  a[^bc]d, [^0-9]

7 Review: The Substitution Operator Similar to the match operator but also enables you to change the matched string.  Use with the binding operator (“ =~ ”) to test whether a variable contains a pattern

8 Review: Changing All Occurrences You can place a g (for global substitution) at the end of the substitution expression to change all occurrences of the target pattern string in the search string. For example,  $name = “tom turtle”;  $name =~ s/t/T/g;  print “Name=$name”; The output of this code would be  Name= Tom TurTle Demo

9 Using Regular Expressions regular expressions to enable programs to match patterns more completely.  They actually make up a small language of special matching operators that can be employed to enhance the Perl string pattern matching.

10 The Alternation Operator Alternation operator looks for alternative strings for matching within a pattern.  (That is, you use it to indicate that the program should match one pattern OR the other). The following shows a match statement using the alternation operator (left) and some possible matches based on the contents of $address (right); this pattern matches either com or edu.

11 Example Alternation Operator

12 Parenthesis For Groupings You use parentheses within regular expressions to specify groupings. For example, the following matches a $name value of Dave or David.

13 Special Character Classes Perl has a special set of character classes for short hand pattern matching For example consider these two statements if ( $name =~ m/ / ) { will match $name with embedded space char if ($name =~ m/\s/ ) { will match $name with embedded space, tab, newline

14 Special Character Classes

15 Special Character Classes - II

16 Special Character Classes - III

17 Setting Specific Patterns w/ Quantifiers Character quantifiers let you look for very specific patterns For example, use the dollar sign (“ $ ”) to to match if a string ends with a specified pattern. if ($ Name =~ /Jones$/ ) { Matches “John Jones” but not “Jones is here” would not. Also, “The guilty party is Jones” would matches.

18 Selected Perl Character Quantifiers I

19 Selected Perl Character Quantifiers II

20 Selected Perl Character Quantifiers III

21 Match the Special Characters Themselves Use a back slash before the special character  \^, \$, \., \?, \(, \), \+, \*,\\,\/ etc  Examples Will a\??bc matches abc, a?bc, bc Will a\++bc matches abc, a+bc, aabc, a++bc

22 Building Regular Expressions That Work Regular expressions are very powerful— but they can also be virtually unreadable.  When building one, tart with a simple regular expression and then refine it incrementally. Build a piece and then test  The following example will build a regular expression for a date checker dd/mm/yyyy format (for example, 05/05/2002 but not 5/12/01 ).

23 1. Determine the precise field rules. - What is valid input and what is not valid input?  E.g., For a date field, think through the valid and invalid rules for the field.  You might allow 09/09/2002 but not 9/9/2002 or Sep/9/2002.  Work through several examples as follows: Building Regular Expressions That Work

24 Work through several examples

25 Building Regular Expressions that Work 2. Get form and form-handling programs working  Build a sending form the input field  Build the receiving program that accepts the field.  For example, a first cut receiving program: $date = param(‘udate’); if ( $date =~ m/.+/ ) { print ‘Valid date=’, $date; } else { print ‘Invalid date=’, $date; } Any Sequence of characters

26 Building Regular Expressions that Work 3. Start with the most specific term possible.  For example, slashes must always separate two characters (for the month), followed by two more characters (for the day), followed by four characters (for the year). if ( $date =~ m{../../....} ) { Any 2 characters Any 2 characters Any 4 characters

27 Building Regular Expressions that Work 4. Anchor and refine. (Use ^ and $ when possible)  if ( $date =~ m{^\d\d/\d\d/\d\d\d\d$} ) { Starts with 2 digits in middle Ends with 4 digits

28 Building Regular Expressions that Work 5. Get more specific if possible.  The first digit of the month can be only 0, 1, 2 or 3. For example, 05/55/2002 is clearly an illegal date.  Only years from this century are allowed. Because we don’t care about dates like 05/05/1999 or 05/05/3003.

29 Add these rules below if ( $date =~ m{^\d\d/[0-3]\d/2\d\d\d$} ) {  Now the regular expression recognizes input like 09/99/2001 and 05/05/4000 as illegal. Year starts with a “2” Month starts with a “0-3” Building Regular Expressions that Work Use { } instead of / / as delimiter

30 Tip: Regular Expression Special Variables Perl regexs set several special scalar variables:  $& will be equal to the first matching text  $` will be the text before the match, and  $’ will be the text after the first match. $name='*****Marty'; if ( $name =~ m/\w/ ) { print "got match at=$& "; print "B4=$` after=$'"; } else { print "Not match"; } would output: got match at=M B4=***** after=arty

31 Full Example Program 1. #!/usr/local/bin/perl 2. use CGI ':standard'; 3. print header, start_html('Date Check'); 4. $date=param('udate'); 5. if ($date =~ m{^\d\d/[0-3]\d/2\d\d\d$}){ 6. print 'Valid date=', $date; 7. } else { 8. print 'Invalid date=', $date; 9. } 10. print end_html;

32 Would Output The Following...

33 CGI Scripting Discussion HW4 Problems  004/winter/ /02/hw4/requirements.txt 004/winter/ /02/hw4/requirements.txt HW5 Problems  004/winter/ /02/hw5/requirements.txt 004/winter/ /02/hw5/requirements.txt