# Regular Expressions. Computer Science and Engineering College of Engineering The Ohio State University. Lecture 9

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1 Regular Expressions Computer Science and Engineering College of Engineering The Ohio State University Lecture 9

2 Language Definition: a set of strings Examples Activity: For each above, find (the cardinality of the set)

3 Programming Languages Q: Are C, Java, Ruby, Python, languages in this formal sense?

4 Programming Languages Q: Are C, Java, Ruby, Python, languages in this formal sense? A: Yes! is the set of well-formed Ruby programs What the interpreter (compiler) accepts The syntax of the language But what does one such string mean? The semantics of the language Not part of formal definition of "language" But necessary to know to claim "I know Ruby"

5 Regular Expression (RE) A formal mechanism for defining a language Precise, unambiguous, well-defined In math, a clear distinction between: Characters in strings (the "alphabet") Meta characters used in writing a RE In computer applications, there isn't Is '*' a Kleene star or an asterisk? (a b)*a(a b)(a b)(a b)

6 Literals A literal represents a character from the alphabet Some are easy: f, i, s, h, Whitespace is hard (invisible!) \t is a tab (ascii 0x09) \n is a newline (ascii 0x0A) \r is a carriage return (ascii 0x0D) So the character '\' needs to be escaped! \\ is a \ (ascii 0x5c)

7 Basic Operators ( ) for grouping, for choice Examples cat dog fish (h H)ello R(uby ails) (G g)r(a e)y These operators are meta-characters too To represent the literal: \( \) \ \(61(3 4)\) Activity: For each RE above, write out the corresponding language explicitly (ie, as a set of strings)

8 Character Class Set of possible characters ( ) is annoying! Syntax: [ ] Explicit list as [ ] Range as [0-9] Negate with ^ at the beginning [^A-Z] a character that is not a capital letter Activity: Write the language defined by Gr[ae]y 0[xX][0-9a-fA-F] [Qq][^u]

9 Character Class Shorthands Common \d for digit, ie [0-9] \s for whitespace, ie [ \t\r\n] \w for word character, ie [0-9a-zA-Z_] And negations too \D, \S, \W (ie [^\d], [^\s], [^\w]) Warning: [^\d\s] [\D\S] POSIX standard (& Ruby) includes [[:alpha:]] alphabetic character [[:lower:]] lowercase alphabetic character [[:digit:]] decimal digit (unicode! Eg ١) [[:xdigit:]] hexadecimal digit [[:space:]] whitespace including newlines

10 Wildcards A. matches any character (almost) Includes space, tab, punctuation, etc! But does not include newline So add. to list of meta-characters Use \. for a literal period Examples Gr.y buckeye\.\d Problem: What is RE for OSU address for everyone named Smith? Answer is not:

11 Repetition Applies to preceding character or ( ) group? means 0 or 1 time * means 0 or more times (unbounded) + means 1 or more times (unbounded) {k} means exactly k times {a,b} means k times, for a k b More meta-characters to escape! \? \* \+ \{ \}

12 Examples colou?r 0[xX](0 [1-9a-fA-F][0-9a-fA-F]*).*\.jpe?g

13 Your Turn (Language consisting of) strings that: Contain only letters, numbers, and _ Start with a letter Do not contain 2 consecutive _'s Do not end with _ Exemplars and counter-exemplars: EOF, 4Temp, Test_Case3, _class, a4_sap_x, S T_2 Write the corresponding RE

14 Your Turn (Language consisting of) strings that: Contain only letters, numbers, and _ Start with a letter Do not contain 2 consecutive _'s Do not end with _ Exemplars and counter-exemplars: EOF, 4Temp, Test_Case3, _class, a4_sap_x, S T_2 Write the corresponding RE

15 Your Turn (Language consisting of) strings that: Contain only letters, numbers, and _ Start with a letter Do not contain 2 consecutive _'s Do not end with _ Exemplars and counter-exemplars: EOF, 4Temp, Test_Case3, _class, a4_sap_x, S T_2 Write the corresponding RE [a-za-z](_[a-za-z0-9] [a-za-z0-9])*

16 Finite State Automota (FSA) An FSA is an "accepting rule" Finite set of states Transition function (relation) between states based on next character in string DFA vs NFA Start state (s 0 ) Set of accepting states An FSA "accepts" a string if you can start in s 0 and end up in an accepting state, consuming 1 character per step

17 Example What language is defined by this FSA? S 0 S 1 0

18 Example What language is defined by this FSA? A. Binary strings (0's and 1's) with an even number of 0's S 0 S 1 0

19 Your Turn (Language consisting of) strings that: Contain only letters, numbers, and _ Start with a letter Do not contain 2 consecutive _'s Do not end with _ Exemplars and counter-exemplars: EOF, 4Temp, Test_Case3, _class, a4_sap_x, S T_2 Write the corresponding FSA

20 Solution a-za-z0-9 a-za-z S 0 S 1 a-za-z0-9 _ S 2

21 Fundamental Results Expressive power of RE is the same as FSA Expressive power of RE is limited Write a RE for "strings of balanced parens" ()(()()), ()(), (((()))), (((, ())((), Can not be done! (impossibility result) Take CSE 3321

22 REs in Practice REs often used to find a "match" A substring s within a longer string such that s is in the language defined by the RE (CSE cse)?3901 Possible uses: Report matching substrings and locations Replace match with something else Practical aspects of using REs this way Anchors Greedy vs lazy matching

23 Anchors Used to specify where matching string should be with respect to a line of text Newlines are natural breaking points ^ anchors to the beginning of a line \$ anchors to the end of a line Ruby: \A \z for beginning/end of string Examples ^Hello World\$ \A[Tt]he ^[^\d].\.jpe?g end\.\z

24 Greedy vs Lazy Repetition (+ and *) allows multiple matches to begin at same place Example: <.*> <h1>title</h1> <h1>title</h1> The match selected depends on whether the repetition matching is greedy, ie matches as much as possible lazy, ie matches as little as possible Default is typically greedy For lazy matching, use *? or +?

25 Regular Expressions in Ruby Instance of a class (Regexp) pattern = Regexp.new('^Rub.') But literal notation is common: /pattern/ /[aeiou]*/ %r{hello+} #no need to escape / Match operator =~ (negation is!~) Operands: String and Regexp (in either order) Returns index of first match (or nil if not present) 'hello world' =~ /o/ #=> 4 /or/ =~ 'hello' #=> nil Options post-pended: /pattern/options i ignore case x ignore whitespace & comments ("free spacing")

26 Strings and Regular Expressions Find all matches as an array a.scan /[[:alpha:]]/ Delimeter for spliting string into array a.split /[aeiou]/ Substitution: sub and gsub (+/-!) Replace first match vs all ("globally") a = "the quick brown fox" a.sub /[aeiou]/, #=> quick brown fox" a.gsub /[aeiou]/, #=>

27 Your Turn (Regular Expressions) Check if phone number in valid format phone = " " #not ok phone = "(614) " #ok format =? if phone? format #well-formatted

28 Summary Language: A set of strings RE: Defines a language Recipe for making elements of language Literals Distinguish characters and metacharacters Character classes Represent 1 character in RE Repetition FSA Expressive power same as RE

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