Unix Regular Expressions

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Regular Expressions Write less, Say more Thursday, January 26, 2012

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The Regular Problem Thursday, January 26, 2012

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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

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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

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Regular Expressions Alternatives Shell Wildcards Dedicated perl/c/java program *.txt x*x[0-9] Thursday, January 26, 2012

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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

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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

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Regular Expressions The Rules Thursday, January 26, 2012

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Rule #1 A Simple character matches itself Thursday, January 26, 2012

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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

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Rule #2 A character class matches a single character from the class Thursday, January 26, 2012

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Character Classes abcdABCD 0123 7 a07 B27 d17 Thursday, January 26, 2012

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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

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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

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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

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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

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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

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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

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Rule #3 A quantiﬁer denotes how many times a letter will match Thursday, January 26, 2012

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Quantiﬁers a + ab b aaab aye cabtain Thursday, January 26, 2012

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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

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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

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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

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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

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Backtracking Examine the expression: [a-z]*b+c Input string: aaaaaaaaaaabbbbbbbbbbbbbcccccccccccc Thursday, January 26, 2012

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Backtracking Examine the expression: [a-z]*b*c Input string: aaaaaaaaaaabbbbbbbbbbbbbcccccccccccc \ Thursday, January 26, 2012

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Rule #4 An assertion will match on a condition, not capturing input characters Thursday, January 26, 2012

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Assertions ^ matches the beginning of a line $ matches the end of a line Thursday, January 26, 2012

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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

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