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Regular expressions

Regular expressions. CS201 Fall 2004 Week 11. Problem. input is very untrustworthy stack smashing, for example lots of data display patterns can we combine these two insights? yes- regular expressions. Example. command line: dir *.java Boo.java Fred.java PainfulClass.java

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Regular expressions

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  1. Regular expressions CS201 Fall 2004 Week 11

  2. Problem • input is very untrustworthy • stack smashing, for example • lots of data display patterns • can we combine these two insights? • yes- regular expressions

  3. Example • command line: dir *.java • Boo.java Fred.java PainfulClass.java • Displays all the java programs in the directory • * - Kleene closure

  4. RE and pattern matching • Web searches • email filtering • text-manipulation (Word) • Perl

  5. How do we use it? • import java.util.regex.*; • specify a pattern • compile it • match • iterate

  6. Specifying Patterns • strings: "To: cwm2n@spamgourmet.com" • can match case exactly • or match case insensitive • Range • [01234567] – any symbol inside the [] • [0-9] • [^j] – caret means "anything BUT j" • one symbol: • . – period manys any character • \\d – a digit, e.g.: [0-9] • \\D – a non-digit [^0-9] • \\w – character, part of a word [a-zA-Z_0-9]

  7. Patterns • quantifier- how many times • * - any number of times (including zero) • .* • ? – zero or one time • A? - A zero or one time • + one or more times • A+ - must find at least one A • others (p. 476)

  8. examples • find subject line of email • "Subject: .*" • finds: Subject: weather • finds: Subject: [POSSIBLE SPAM] get a degree! • Problem • also finds • How to be a British Subject: marry into the Royal

  9. Anchors • tell us where to find what we are looking for • ^ - beginning of line • ^Subject: .* • $ - end of line • ^com • others on page 478

  10. Alternation • subject line either SPAM or Rolex • ^Subject:.*(SPAM.* | Rolex.*)

  11. How to use it, really • Form a pattern • Pattern p = Pattern.compile("^Subject: .*"); • Create a Matcher • Matcher m = p.matcher(someBuffer); • iterate while(m.find()) System.out.println("Found text: "+m.group()); • find()- boolean, next occurence found • group() – String that matches

  12. example package edu.virginia.cs.cs201.fall04; import java.util.regex.*; public class Tryout { String text = "A horse is a horse, of course of course.."; String pattern = "horse|course"; public static void main(String args[]) { Tryout t = new Tryout(); t.go(); } public void go() { Pattern p = Pattern.compile(pattern); Matcher m = p.matcher(text); while(m.find()) { System.out.println(m.group()+m.start()); } } } horse2 horse13 course23 course33

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