Eloquent Ruby chapter 4 - Find The Right String with Regular Expression

CHAPTER 5
Find the Right String with
Regular Expressions
 Matching One Character at a Time
 Sets, Ranges, and Alternatives
 The Regular Expression Star
 Regular Expressions in Ruby
 Beginnings and Endings
 In the Wild
 Staying Out of Trouble

● Matching One Character at a Time
 letters and numbers match themselves
 Regular expressions is case sensitive. (default)
 Unlike letters and numbers, most of the punctuation
characters—things like . and *—have special
meanings in regular expressions. ( e.g the period
or dot character matches any single character
except a newline character)
 You use a backslash to turn off the special
meanings of the punctuation characters

●Sets, Ranges, and Alternatives
 To create a regular expression set, you wrap the
characters in square brackets
 A Range by specifying the beginning and end of a
sequence of characters, separated by a dash
 Shortcut for range:
 d will match any digit
 w, where the w stands for “word character,” will
match any letter, number or the underscore.
 s will match any white space character including the
vanilla space, the tab, and the newline.

The Regular Expression Star
 In regular expressions, an asterisk (*) matches zero
or more of the thing that came just before it
 A* , AB*, R*uby, R*u*by
 [aeiou]*, [09]*, [09af]*,
 .* , .. , .*George, .*George.*

Regular Expressions in Ruby
 In Ruby, the regular expression, or Regexp for
short, is one of the builtin data types, with its
own special literal syntax. Encase pattern
between forward slashes. (like JavaScript)
 You use the =~ operator to test whether a regular
expression matches a string. return char index
when match and nil when not.
class Regexp
def test(str)
(self =~ str) ? false : true
end
end

Regular Expressions in Ruby (cont')
 Turn case sensitivity off my sticking an i on the end
of your expression
 Regular expressions also come into play in the
string methods that involve searching e.g. 'sub',
'gsub'
 @content.gsub!( /dd:dd (AM|PM)/, '**:** **' )


Beginnings and Endings
 A (capital letter A) match the beginning or
circumflex ^
 z (note the lower case) or sign $ matches the end of
the string
 The .* won’t match across the lines . . . unless we
simply turn off this behavior by adding an m to
our expression
 e.g /^Once upon a time.*happily ever after.$/


In the Wild
 ? The question mark matches zero or one of the
things


Bonus
 '^' in Set vs '^' for beginning. e.g. /[^aeiou]/,
/^[aeiou]
 d & D, w & W
 +, {n}, {n,},{n,n}, # n is number
 $n # n is number
 n # n is number in string with single quote
 String.match reg # == reg.match String
 '1234567890' => (123) 4567890
 '123456(789)[0]' => (123) 4567890

References
 Addison Wesley Eloquent Ruby
 http://net.tutsplus.com/tutorials/ruby/rubyfornewbiesregularexpressions/
 http://www.w3schools.com/jsref/jsref_obj_regexp.asp
 http://www.tutorialspoint.com/ruby/ruby_regular_expressions.htm


Eloquent Ruby chapter 4 - Find The Right String with Regular Expression

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