Unix Regular Expressions

Transcript

1. Regular Expressions Write less, Say more Thursday, January 26, 2012
2. The Regular Problem Thursday, January 26, 2012
3. The Regular Problem Regular Expressions provide a language that describes text and other languages It’s a mini language of its own, with syntax rules Regular Expressions are a power tool for solving text related problems Thursday, January 26, 2012
4. The Regexp Story Started in Mathematics 1968 Entered the Unix world through Ken Thompson’s qed 1984 Standardized by Henry Spencer’s implementation Thursday, January 26, 2012
5. Regular Expressions Alternatives Shell Wildcards Dedicated perl/c/java program *.txt x*x[0-9] Thursday, January 26, 2012
6. Regular Expressions & Unix Many UNIX tools take regular expressions grep/egrep filters its input based on regular expressions more/less/most search uses regular expressions vi/vim search and replace use regular expressions Thursday, January 26, 2012
7. Regular Expressions Today Used by all programming languages, including: Php, Python, Perl, Tcl JavaScript, ActionScript, Microsoft .NET, Oracle Java Objective C And More Thursday, January 26, 2012
8. Regular Expressions The Rules Thursday, January 26, 2012
9. Rule #1 A Simple character matches itself Thursday, January 26, 2012
10. Examples Command Meaning egrep foo Display only input lines that include the word ‘foo’ egrep unix Display only input lines that include the word ‘unix’ Thursday, January 26, 2012
11. Rule #2 A character class matches a single character from the class Thursday, January 26, 2012
12. Character Classes abcdABCD 0123 7 a07 B27 d17 Thursday, January 26, 2012
13. Character Class Syntax A class is denoted by [...] Can use any character sequence inside the squares [012], [abc], [aAbBcZ] Can use ranges inside the squares [0-9], [a-z], [a-zA-Z], [0-9ab] Can use not [^abc], [^0-9] Thursday, January 26, 2012
14. Examples Command Meaning egrep ‘[0-9][0-9]’ Display only input lines that include at least two digits egrep ‘[Uu][Nn][Ii][Xx]’ Display only input lines that include the word ‘unix’ in any casing Thursday, January 26, 2012
15. Which of these match ? hello [ux][012] hello world hello [ux][012] hello unix hello [ux][012] hello u2 hello [ux][012] hello x10 hello [ux][012] HELLO U2 Thursday, January 26, 2012
16. Which of these match ? hello [ux][012] hello world hello [ux][012] hello uni0 hello [ux][012] hello u2 hello [ux][012] hello x10 hello [ux][012] HELLO U2 Thursday, January 26, 2012
17. Predefined Character Classes [:alnum:] [:alpha:] [:cntrl:] [:digit:] [:graph:] [:lower:] [:print:] [:punct:] [:space:] [:upper:] [:xdigit:] . cheat sheet at: http://www.petefreitag.com/cheatsheets/regex/character-classes/ Thursday, January 26, 2012
18. Predefined Character Classes Note that brackets are part of the class name, therefore the correct use is: [[:digit:]] This allows using: [[:digit:][:lower:]] Thursday, January 26, 2012
19. Rule #3 A quantifier denotes how many times a letter will match Thursday, January 26, 2012
20. Quantifiers a + ab b aaab aye cabtain Thursday, January 26, 2012
21. Quantifiers Syntax * means match zero or more times - {0,} + means match one or more times - {1,} ? means match zero or one time - {0,1} {n,m} means match at least n but no more than m times {n} means match exactly n times Thursday, January 26, 2012
22. Which of these match ? [[:digit:]]{2}-?[[:digit:]]{7} 08-9112232 [[:digit:]]{2}-?[[:digit:]]{7} 421121212 [[:digit:]]{2}-?[[:digit:]]{7} 054-2201121 [[:digit:]]{2}-?[[:digit:]]{7} Phone: 03-9112121 [[:digit:]]{2}-?[[:digit:]]{7} Bond 007 Thursday, January 26, 2012
23. Which of these match ? [[:digit:]]{2}-?[[:digit:]]{7} 08-9112232 [[:digit:]]{2}-?[[:digit:]]{7} 421121212 [[:digit:]]{2}-?[[:digit:]]{7} 054-2201121 [[:digit:]]{2}-?[[:digit:]]{7} Phone: 03-9112121 [[:digit:]]{2}-?[[:digit:]]{7} Bond 007 Thursday, January 26, 2012
24. Which of these match ? (http://)?w{3}\.[a-z]+\.com www.google.com (http://)?w{3}\.[a-z]+\.com www.ynet.co.il (http://)?w{3}\.[a-z]+\.com http://mail.google.com (http://)?w{3}\.[a-z]+\.com http://www.home.com (http://)?w{3}\.[a-z]+\.com http://www.tel-aviv.com Thursday, January 26, 2012
25. Which of these match ? (http://)?w{3}\.[a-z]+\.com www.google.com (http://)?w{3}\.[a-z]+\.com www.ynet.co.il (http://)?w{3}\.[a-z]+\.com http://mail.google.com (http://)?w{3}\.[a-z]+\.com http://www.home.com (http://)?w{3}\.[a-z]+\.com http://www.tel-aviv.com Thursday, January 26, 2012
26. Backtracking When the engine encounters a quantifier, it will keep on adding matches to the quantified element as long as possible If a match failure occurs later on, the engine will backtrack Thursday, January 26, 2012
27. Backtracking Examine the expression: [a-z]*b+c Input string: aaaaaaaaaaabbbbbbbbbbbbbcccccccccccc Thursday, January 26, 2012
28. Backtracking Examine the expression: [a-z]*b*c Input string: aaaaaaaaaaabbbbbbbbbbbbbcccccccccccc \ Thursday, January 26, 2012
29. Rule #4 An assertion will match on a condition, not capturing input characters Thursday, January 26, 2012
30. Assertions ^ matches the beginning of a line $ matches the end of a line Thursday, January 26, 2012
31. Which of these match ? ^d drwxr-xr-x dive ^d -rwxr-xr-x dive ^d lrwxr-xr-x dive ^d drwxr-xr-x /home ^d -rwxr-xr-x /etc/passwd Thursday, January 26, 2012
32. Which of these match ? ^d drwxr-xr-x dive ^d -rwxr-xr-x dive ^d lrwxr-xr-x dive ^d drwxr-xr-x /home ^d -rwxr-xr-x /etc/passwd Thursday, January 26, 2012
33. Regular Expressions Variants Old style regexps didn’t have a + or {}. Therefore, in grep, these have to be backslashed Use egrep when possible for cleaner syntax Thursday, January 26, 2012
34. Q & A Thursday, January 26, 2012
35. Thank You Ynon Perek ynonperek@hotmail.com ynonperek.com Thursday, January 26, 2012