Perl Regular Expressions

Working with regular expressions in perl

Transcript

1. Regular Expressions Write less, Say more Thursday, December 22, 2011
2. The Regular Problem Thursday, December 22, 2011
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, December 22, 2011
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, December 22, 2011
5. Regular Expressions Alternatives Shell Wildcards Dedicated perl/c/java program *.txt x*x[0-9] Thursday, December 22, 2011
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, December 22, 2011
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, December 22, 2011
8. Regular Expressions The Rules Thursday, December 22, 2011
9. Rule #1 A Simple character matches itself Thursday, December 22, 2011
10. Examples Expression Meaning foo Match only input lines that include the word ‘foo’ unix Match only input lines that include the word ‘unix’ Thursday, December 22, 2011
11. Rule #2 A character class matches a single character from the class Thursday, December 22, 2011
12. Character Classes abcdABCD 0123 7 a07 B27 d17 Thursday, December 22, 2011
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] Thursday, December 22, 2011
14. Examples Expression Meaning [0-9][0-9] Match only input lines that include at least two digits [Uu][Nn][Ii][Xx] Match only input lines that include the word ‘unix’ in any casing Thursday, December 22, 2011
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, December 22, 2011
16. 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, December 22, 2011
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, December 22, 2011
18. Predefined Character Classes \w (\W) - match [0-9a-zA-Z_] (or other) \s (\S) - match a white space (or other) \d (\D) - match a digit (or other) cheat sheet at: http://www.petefreitag.com/cheatsheets/regex/character-classes/ Thursday, December 22, 2011
19. Predefined Character Classes Note that brackets are part of the class name, therefore the correct use is: [[:digit:]] This allows using: [[:digit:][:lower:]] Thursday, December 22, 2011
20. Rule #3 A quantifier denotes how many times a letter will match Thursday, December 22, 2011
21. Quantifiers a + ab b aaab aye cabtain Thursday, December 22, 2011
22. Quantifiers Syntax * means match zero or more times + means match one or more times ? means match zero or one time {n,m} means match at least n but no more than m times {n} means match exactly n times Thursday, December 22, 2011
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, December 22, 2011
24. 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, December 22, 2011
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, December 22, 2011
26. 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, December 22, 2011
27. 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, December 22, 2011
28. Backtracking Examine the expression: [a-z]*b+c Input string: aaaaaaaaaaabbbbbbbbbbbbbcccccccccccc Thursday, December 22, 2011
29. Backtracking Examine the expression: [a-z]*b*c Input string: aaaaaaaaaaabbbbbbbbbbbbbcccccccccccc \ Thursday, December 22, 2011
30. Rule #4 An assertion will match on a condition, not capturing input characters Thursday, December 22, 2011
31. Assertions ^ matches the beginning of a line $ matches the end of a line \b matches word boundary Thursday, December 22, 2011
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, December 22, 2011
33. 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, December 22, 2011
34. Which of these match ? ^.$ x ^.$ mmm ^.$ 42 ^.$ 9 ^.$ ... Thursday, December 22, 2011
35. Which of these match ? ^.$ x ^.$ mmm ^.$ 42 ^.$ 9 ^.$ ... Thursday, December 22, 2011
36. Captures Thursday, December 22, 2011
37. Capturing Parens Use parens to capture matched expression Use \1, \2, etc. to refer to captured match Thursday, December 22, 2011
38. Capturing Parens Paris in the the spring (\b\w+\b) \1 Thursday, December 22, 2011
39. Which of these match ? (\d)(\d)\2\1 1111 (\d)(\d)\2\1 1001 (\d)(\d)\2\1 1414 (\d)(\d)\2\1 12321 (\d)(\d)\2\1 9889 Thursday, December 22, 2011
40. Which of these match ? (\d)(\d)\2\1 1111 (\d)(\d)\2\1 1001 (\d)(\d)\2\1 1414 (\d)(\d)\2\1 12321 (\d)(\d)\2\1 9889 Thursday, December 22, 2011
41. Q & A Regular Expressions Classes Quantifiers Assertions Captures Thursday, December 22, 2011
42. Let’s Talk Perl Thursday, December 22, 2011
43. Define A Regexp use qr to define a regular expression my $DIGITS_RE = qr { ^ \d+ $ }xms; xms make the regexp more readable, and work better on multiline strings Thursday, December 22, 2011
44. Match Against a Regexp $text =~ $DIGITS_RE returns true if $text matches the pattern Can also use inline regexp: $text =~ /^\d+$/; Thursday, December 22, 2011
45. Match With Capture If a regexp has captures, the return value of the match operator is a list of the captured groups my ($key, $value) = $line =~ $CONFIG_LINE; Thursday, December 22, 2011
46. Search & Replace Use s/// to perform a search & replace operation replace first occurrence of $PATTERN in $text with contents of $new: $text =~ s/$PATTERN/$new/; Thursday, December 22, 2011
47. Search & Replace replace all occurrence of $PATTERN in $text with contents of $new: $text =~ s/$PATTERN/$new/g; Thursday, December 22, 2011
48. Q & A Thursday, December 22, 2011
49. Thank You Ynon Perek ynonperek@yahoo.com ynonperek.com All Rights Reserved to Ynon Perek Thursday, December 22, 2011