Many of us approach regular expressions with a certain fear and trepidation, using them only when absolutely necessary. We can get by when we need to use them, but we hesitate to dive any deeper into their cryptic world. Ruby has so much more to offer us through the Regexp class and the Oniguruma regular expression library. This talk will showcase advanced regular expression techniques include grouping, lookahead and lookbehinds. Are your regular expressions greedy, lazy, or possessive? Learn how to change their behavior for the better. You will walk away with tools and techniques to harness the power of Ruby regular expressions and bring beauty and elegance into your code.

1. BEYOND THE BASICS
Regular Expressions in Ruby
@nellshamrell
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2. ^4[0-9]{12}(?:[0-9]{3})?$
Source: regular-expressions.info
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3. Regular Expressions are Patterns
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4. Test
Extract
Change
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5. Test
Extract
Change
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6. Test
Extract
Change
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7. In Ruby, regular
expressions are objects
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8. You program from a regular
expression to a result
Source: The Well Grounded Rubyist
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9. Oniguruma
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10. Shorthand for Hexadecimals
/h and /H
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11. Onigmo
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12. Thursday, February 7, 13

13. =~
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14. /force/ =~ “Use the force”
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15. “Use the force” =~ /force/
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16. “Use the force” =~ /force/
=> 8
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17. /dark side/ !~ “Use the force”
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18. /dark side/ !~ “Use the force”
=> true
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19. MatchData
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20. .match
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21. string = “The force will be with
you always.”
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22. string = “The force will be with
you always.”
m =/force/.match(string)
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23. string = “The force will be with
you always.”
m =/force/.match(string)
=> #<MatchData “force” >
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24. string = “The force will be with
you always.”
m =/force/.match(string,5)
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25. string = “The force will be with
you always.”
m =/force/.match(string,5)
=> nil
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26. What can you do with MatchData?
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27. m.to_s
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28. m.to_s
=> “force”
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29. m.to_s
=> “force”
m.pre_match
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30. m.to_s
=> “force”
m.pre_match
=> “The ”
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31. m.to_s
=> “force”
m.pre_match
=> “The ”
m.post_match
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32. m.to_s
=> “force”
m.pre_match
=> “The ”
m.post_match
=> “ be with you”
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33. Capture Groups
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34. /(.*)force(.*)/
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35. m = /(.*)force(.*)/.match(string)
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36. m = /(.*)force(.*)/.match(string)
m.captures
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37. m = /(.*)force(.*)/.match(string)
m.captures
=> [“The ”,
“will be with you always”]
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38. You access capture groups with []
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39. m[1]
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40. m[1]
=> “The ”
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41. m[1]
=> “The ”
m[2]
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42. m[1]
=> “The ”
m[2]
=> “ will be with you always ”
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43. m[0]
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44. m[0]
=> “The force will be with
you always.”
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45. Match Objects are not arrays
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46. m.each do |match|
puts match.upcase
end
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47. m.each do |match|
puts match.upcase
end
=> NoMethodError
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48. m.to_a.each do |match|
puts match.upcase
end
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49. m.to_a.each do |match|
puts match.upcase
end
=> “THE FORCE WILL BE
WITH YOU ALWAYS”
“THE ”
“WILL BE WITH YOU
ALWAYS”
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50. Thursday, February 7, 13

51. LookArounds Define Context
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52. string = “Who’s the more
foolish? The fool or
the fool who follows
him?”
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53. string = “Who’s the more
foolish? The fool or
the fool who follows
him?”
/fool/
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54. string.scan(/fool/)
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55. string.scan(/fool/)
=> [“fool”, “fool”, “fool”]
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56. Positive Lookahead
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57. ?=
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58. string.scan(/fool(?=ish)/)
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59. string.scan(/fool(?=ish)/)
=> [“fool”]
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60. string.gsub(/fool(?=ish)/, “self”)
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61. string.gsub(/fool(?=ish)/, “self”)
=> “Who’s the more
selfish? The fool or
the fool who follows
him?”
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62. Zero Width Positive
Lookahead Assertion
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63. Zero width means it does not
consume characters
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64. Positive means a match for the
lookahead should be present
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65. Lookahead means it is looking
ahead of your main match.
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66. Assertion means the lookahead only
determines whether a match exists
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67. string = “Who’s the more
foolish? The fool or
the fool who follows
him?”
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68. Negative Lookahead
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69. Negative means a match for the
lookahead should not be present
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70. ?!
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71. string.scan(/fool(?!ish)/)
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72. string.scan(/fool(?!ish)/)
=> [“fool”, “fool”]
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73. string.gsub(/fool(?!ish)/, “self”)
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74. string.gsub(/fool(?!ish)/, “self”)
=> “Who’s the more
foolish? The self or
the self who follows
him?”
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75. Positive Lookbehind
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76. string = “For my ally is the
force, and a powerful
ally it is”
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77. string = “For my ally is the
force, and a powerful
ally it is”
/ally/
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78. ?<=
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79. /(?<=powerful )ally/
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80. string.gsub(/(?<=powerful )ally/,
“friend”)
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81. string.gsub(/(?<=powerful )ally/,
“friend”)
=> “For my ally is the
force, and a powerful
friend it is”
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82. Negative Lookbehind
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83. ?<!
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84. /(?<!powerful )ally/
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85. string.gsub(/(?<!powerful )ally/,
“friend”)
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86. string.gsub(/(?<!powerful )ally/,
“friend”)
=> “For my friend is the
force, and a powerful
ally it is”
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87. Thursday, February 7, 13

88. Regular Expressions have
distinct behaviors
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89. Greedy
Lazy
Possessive
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90. Greedy
Lazy
Possessive
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91. Greedy
Lazy
Possessive
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92. Quantifiers
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93. +
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94. +
/.+/
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95. Quantifiers are greedy by default
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96. Greedy Quantifiers match
as much as possible
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97. Greedy Quantifiers use maximum
effort for maximum return
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98. string = “This is no time to
talk about time we
don’t have the time”
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99. string = “This is no time to
talk about time we
don’t have the time”
/.+time/
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100. /.+time/.match(string)
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101. /.+time/.match(string)
=> “This is no time to
talk about time we
don’t have the time”
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102. Greedy regular expressions
try to match the whole
string, then backtrack
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103. Oniguruma makes
backtracking quicker
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104. Lazy Quantifiers
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105. Lazy Quantifiers match
as little as possible
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106. Lazy Quantifiers use minimum
effort for minimum return
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107. /.+?time/
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108. /.+?time/.match(string)
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109. /.+?time/.match(string)
=> “This is no time”
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110. Lazy regular expressions
use less resources
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111. Possessive Quantifiers
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112. Possessive Quantifiers are
all or nothing
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113. Possessive Quantifiers try to
match the entire string
with no backtracking
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114. Possessive Quantifiers use
minimum effort for
maximum return
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115. /.++time/
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116. /.++time/.match(string)
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117. /.++time/.match(string)
=> nil
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118. Possessive Quantifiers
fail faster
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119. Thursday, February 7, 13

120. Write regular expressions
in small chunks
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121. Rubular
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122. Regular expressions come in drafts
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123. Move beyond your fear
of regular expressions
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124. Nell Shamrell
Software Development Engineer
Blue Box Group
@nellshamrell
Thursday, February 7, 13