This document provides an introduction to regular expressions (regexes). It discusses that regexes are often feared by programmers due to their mysterious nature. It then defines what a regex is, as a small language for describing text that can be used to find and replace text. The document recommends learning regexes because they are heavily used in Ruby and are fast and encoding aware. It provides examples of basic regex usage in Ruby strings, including finding, replacing, and modifying strings. It also covers literal characters, character classes, shortcut character classes, and anchors in regexes.

1. Regular Expressions
The Black Magic of Programming

2. The Basics

3. The Fear Factor!

4. The Fear Factor!
For unknown reasons regular expressions
are deeply shrouded in mystery

5. The Fear Factor!
For unknown reasons regular expressions
are deeply shrouded in mystery
Many programmers outright fear them

6. The Fear Factor!
For unknown reasons regular expressions
are deeply shrouded in mystery
Many programmers outright fear them
I stumped a room full of programmers
in Tulsa by shouting out a two
character expression

7. The Fear Factor!
For unknown reasons regular expressions
are deeply shrouded in mystery
Many programmers outright fear them
I stumped a room full of programmers
in Tulsa by shouting out a two
character expression
I have know idea why this is

8. What is a Regex?

9. What is a Regex?
Regular expression is a very small
language for describing text

10. What is a Regex?
Regular expression is a very small
language for describing text
You can use them to dissect and change
textual data

11. What is a Regex?
Regular expression is a very small
language for describing text
You can use them to dissect and change
textual data
I think of them as a DSL for find and
replace operations

12. Why Learn Regular Expressions?

13. Why Learn Regular Expressions?
Ruby leans heavily on regular expressions:

14. Why Learn Regular Expressions?
Ruby leans heavily on regular expressions:
Many text operations in Ruby are
easiest with the right regex

15. Why Learn Regular Expressions?
Ruby leans heavily on regular expressions:
Many text operations in Ruby are
easiest with the right regex
Regular expressions are fast

16. Why Learn Regular Expressions?
Ruby leans heavily on regular expressions:
Many text operations in Ruby are
easiest with the right regex
Regular expressions are fast
Regular expressions are encoding aware

17. Why Learn Regular Expressions?
Ruby leans heavily on regular expressions:
Many text operations in Ruby are
easiest with the right regex
Regular expressions are fast
Regular expressions are encoding aware
You can be the one scaring all the other
programmers

18. Basic Regex Usage

19. Basic Regex Usage
Strings has methods
supporting:

20. Basic Regex Usage
Strings has methods
supporting:
Find/Find All

21. Basic Regex Usage
Strings has methods
supporting:
Find/Find All
Replace/Replace All

22. Basic Regex Usage
Strings has methods
supporting:
Find/Find All
Replace/Replace All
Use sub!()/gsub!() to
modify a String in
place

23. Basic Regex Usage
Strings has methods if "100" =~ /Ad+z/
supporting: puts "This is a number."
end
Find/Find All
Replace/Replace All
Use sub!()/gsub!() to
modify a String in
place

24. Basic Regex Usage
Strings has methods if "100" =~ /Ad+z/
supporting: puts "This is a number."
end
"Find all, words.".scan(/w+/) do |word|
Find/Find All puts word.downcase
end
year, month, day = "2008-09-04".scan(/d+/)
Replace/Replace All
Use sub!()/gsub!() to
modify a String in
place

25. Basic Regex Usage
Strings has methods if "100" =~ /Ad+z/
supporting: puts "This is a number."
end
"Find all, words.".scan(/w+/) do |word|
Find/Find All puts word.downcase
end
year, month, day = "2008-09-04".scan(/d+/)
Replace/Replace All
csv = "C, S, V".sub(/,s+/, ",")
cap = "one two".sub(/w+/) { |n| n.capitalize }
Use sub!()/gsub!() to
modify a String in
place

26. Basic Regex Usage
Strings has methods if "100" =~ /Ad+z/
supporting: puts "This is a number."
end
"Find all, words.".scan(/w+/) do |word|
Find/Find All puts word.downcase
end
year, month, day = "2008-09-04".scan(/d+/)
Replace/Replace All
csv = "C, S, V".sub(/,s+/, ",")
cap = "one two".sub(/w+/) { |n| n.capitalize }
Use sub!()/gsub!() to
modify a String in csv = "C, S, V".gsub(/,s+/, ",")
caps = "one two".gsub(/w+/) { |n| n.capitalize }
place

27. Literal Characters

28. Literal Characters
Most characters in a regex match
themselves literally

29. Literal Characters
Most characters in a regex match
themselves literally
The only special characters are:
[].^$?*+{}|()

30. Literal Characters
Most characters in a regex match
themselves literally
The only special characters are:
[].^$?*+{}|()
You can proceed a special character
with to make it literal

31. Literal Characters
Most characters in a regex match
themselves literally
The only special characters are:
[].^$?*+{}|()
You can proceed a special character
with to make it literal
The regex /James Gray/ matches my name

32. Character Classes

33. Character Classes
Characters in [ … ] are choices for a
single character match

34. Character Classes
Characters in [ … ] are choices for a
single character match
A leading ^ negates the class, so [^ … ]
matches what is not listed

35. Character Classes
Characters in [ … ] are choices for a
single character match
A leading ^ negates the class, so [^ … ]
matches what is not listed
You can use ranges like a-z or 0-9

36. Character Classes
Characters in [ … ] are choices for a
single character match
A leading ^ negates the class, so [^ … ]
matches what is not listed
You can use ranges like a-z or 0-9
The expression /[bcr]at/ will match
“bat,” “cat,” or “rat”

37. Shortcut Character Classes

38. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

39. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

40. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

41. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

42. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

43. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

44. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

45. Shortcut Character Classes
Shortcut Actual Character Class
. [^n]
s [ tnrfv]
S [^ tnrfv]
w [a-zA-Z0-9_]
W [^a-zA-Z0-9_]
d [0-9]
D [^0-9]

46. Anchors

47. Anchors
Anchors match
between characters

48. Anchors
Anchors match
between characters
They are used to assert
that the content you
want must appear in a
certain place

49. Anchors
Anchors match
between characters
They are used to assert
that the content you
want must appear in a
certain place
Thus /^Totals/ searches
for a line starting with
“Totals”

50. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

51. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

52. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

53. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

54. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

55. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

56. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

57. Anchors
Anchor Matches
Anchors match
between characters A Start of the String
End of the String or
Z
They are used to assert before trailing newline
that the content you z End of the String
want must appear in a ^ Start of a line
certain place
$ End of a line
Thus /^Totals/ searches Between wW or Ww,
b
for a line starting with and at A and z
“Totals” B Between ww or WW

58. Repetition

59. Repetition
You can tack symbols
onto an element of a
regex to indicate that
element can repeat

60. Repetition
You can tack symbols
onto an element of a
regex to indicate that
element can repeat
The expression /ab+c?/
matches an a, followed
by one or more b’s, and
optionally followed by
ac

61. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

62. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

63. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

64. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

65. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

66. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

67. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

68. Repetition
You can tack symbols Repeater Allowed Count
onto an element of a
? Zero or one
regex to indicate that
element can repeat + One or more
* Zero or more
The expression /ab+c?/ {n} Exactly n
matches an a, followed {n,} At least n
by one or more b’s, and
{,m} No more than m
optionally followed by
ac {n,m} Between n and m

69. Some Examples

70. Some Examples
if var =~ /As*z/
puts "Variable is blank."
end

71. Some Examples
if var =~ /As*z/
puts "Variable is blank."
end
if var !~ /S/
puts "Variable is blank."
end

72. Some Examples
if var =~ /As*z/
puts "Variable is blank."
end
if var !~ /S/
puts "Variable is blank."
end
From TopCoder.com, SRM 216 “CultureShock:”
Bob and Doug have recently moved from Canada to the United States, and they are confused
by this strange letter, "ZEE". They need your assistance. Given a String text, replace every
occurrence of the word, "ZEE", with the word, "ZED", and return the result.
Note that if "ZEE" is just part of a larger word (for example, "ZEES"), it should not be altered.

73. Some Examples
if var =~ /As*z/
puts "Variable is blank."
end
if var !~ /S/
puts "Variable is blank."
end
From TopCoder.com, SRM 216 “CultureShock:”
Bob and Doug have recently moved from Canada to the United States, and they are confused
by this strange letter, "ZEE". They need your assistance. Given a String text, replace every
occurrence of the word, "ZEE", with the word, "ZED", and return the result.
Note that if "ZEE" is just part of a larger word (for example, "ZEES"), it should not be altered.
solution = text.gsub(/bZEEb/, "ZED")

74. Greedy Verses Non-Greedy

75. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many
characters as possible

76. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many
characters as possible
The match will
backtrack, giving
up characters, if it
helps it succeed

77. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many
characters as possible
The match will
backtrack, giving
up characters, if it
helps it succeed
You can negate this,
matching minimal
characters

78. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

79. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

80. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

81. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

82. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

83. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

84. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

85. Greedy Verses Non-Greedy
By default repetition
will always be greedy,
consuming as many Greedy Non-Greedy
characters as possible ? ??
+ +?
The match will
* *?
backtrack, giving
up characters, if it {n} N/A
helps it succeed {n,} {n,}?
{,m} {,m}?
You can negate this, {n,m} {n,m}?
matching minimal
characters

86. Alternation

87. Alternation
In a regex, | means “or”

88. Alternation
In a regex, | means “or”
You can put a full expression on the left
and another full expression on the right

89. Alternation
In a regex, | means “or”
You can put a full expression on the left
and another full expression on the right
Either can match

90. Alternation
In a regex, | means “or”
You can put a full expression on the left
and another full expression on the right
Either can match
The expression /James|words?/ will
match “James,” “word,” or “words”

91. Grouping

92. Grouping
Everything in ( … ) is grouped into a
single element for the purposes of
repetition and alternation

93. Grouping
Everything in ( … ) is grouped into a
single element for the purposes of
repetition and alternation
The expression /(ha)+/ matches “ha,”
“haha,” “hahaha,” etc.

94. Grouping
Everything in ( … ) is grouped into a
single element for the purposes of
repetition and alternation
The expression /(ha)+/ matches “ha,”
“haha,” “hahaha,” etc.
The expression /Greg(ory)?/ matches
“Greg” and “Gregory”

95. Captures

96. Captures
( … ) also capture
what they match

97. Captures
( … ) also capture
what they match
After a match, you can
access these captures
in the variables $1, $2,
etc., from left to right

98. Captures
( … ) also capture
what they match
After a match, you can
access these captures
in the variables $1, $2,
etc., from left to right
Use 1, 2, etc. in
String replacements

99. Captures
( … ) also capture
what they match
"$99.95" =~ /$(d+(.d+)?)/
After a match, you can
access these captures
in the variables $1, $2,
etc., from left to right
Use 1, 2, etc. in
String replacements

100. Captures
( … ) also capture
what they match
"$99.95" =~ /$(d+(.d+)?)/
After a match, you can
access these captures
in the variables $1, $2,
$1
etc., from left to right
Use 1, 2, etc. in
String replacements

101. Captures
( … ) also capture
what they match
"$99.95" =~ /$(d+(.d+)?)/
After a match, you can
access these captures
in the variables $1, $2,
$1
etc., from left to right $2
Use 1, 2, etc. in
String replacements

102. Modes

103. Modes
Regular expressions have modes

104. Modes
Regular expressions have modes
End an expression with /i to make the
expression case insensitive

105. Modes
Regular expressions have modes
End an expression with /i to make the
expression case insensitive
End with /m for “multi-line” mode
where . will also match newlines

106. Modes
Regular expressions have modes
End an expression with /i to make the
expression case insensitive
End with /m for “multi-line” mode
where . will also match newlines
Use /x to add space and comments

107. Modes
Regular expressions have modes
End an expression with /i to make the
expression case insensitive
End with /m for “multi-line” mode
where . will also match newlines
Use /x to add space and comments
You can combine modes: /mi

108. More Examples

109. More Examples
if ip =~ /Ad{1,3}(.d{1,3}){3}z/
puts "IP adress is well formed."
end

110. More Examples
if ip =~ /Ad{1,3}(.d{1,3}){3}z/
puts "IP adress is well formed."
end
if text =~ /b(at|for|in)[.?!]/
puts "You have bad grammar."
end

111. More Examples
if ip =~ /Ad{1,3}(.d{1,3}){3}z/
puts "IP adress is well formed."
end
if text =~ /b(at|for|in)[.?!]/
puts "You have bad grammar."
end
james_gray = "Gray, James".sub(/(S+),s*(.+)/, '2 1')

112. Other Tricks

113. Other Tricks
There are other special
variables for regexen
including $`, $&, and $’

114. Other Tricks
There are other special
variables for regexen
including $`, $&, and $’
You can escape content
for use in a regex

115. Other Tricks
There are other special
variables for regexen
including $`, $&, and $’
You can escape content
for use in a regex
There’s a MatchData
object for matches

116. Other Tricks
There are other special
variables for regexen
including $`, $&, and $’
You can escape content
for use in a regex
There’s a MatchData
object for matches
Many methods can take
a regex

117. Other Tricks
"one_two_three" =~ /two/
There are other special one_, two, _three = $`, $&, $'
variables for regexen
including $`, $&, and $’
You can escape content
for use in a regex
There’s a MatchData
object for matches
Many methods can take
a regex

118. Other Tricks
"one_two_three" =~ /two/
There are other special one_, two, _three = $`, $&, $'
variables for regexen
print "What's your favorite language? "
including $`, $&, and $’ lang = $stdin.gets.strip
if "Perl Java" =~ /b#{Regexp.escape(lang)}b/i
puts "You are weird."
else
You can escape content puts "OK."
end
for use in a regex
There’s a MatchData
object for matches
Many methods can take
a regex

119. Other Tricks
"one_two_three" =~ /two/
There are other special one_, two, _three = $`, $&, $'
variables for regexen
print "What's your favorite language? "
including $`, $&, and $’ lang = $stdin.gets.strip
if "Perl Java" =~ /b#{Regexp.escape(lang)}b/i
puts "You are weird."
else
You can escape content puts "OK."
end
for use in a regex
CONFIG_RE = /A([^=s]+)s*=s*(S+)/
config = "url = http://ruby-lang.org"
There’s a MatchData key, value = config.match(CONFIG_RE).captures
object for matches
Many methods can take
a regex

120. Other Tricks
"one_two_three" =~ /two/
There are other special one_, two, _three = $`, $&, $'
variables for regexen
print "What's your favorite language? "
including $`, $&, and $’ lang = $stdin.gets.strip
if "Perl Java" =~ /b#{Regexp.escape(lang)}b/i
puts "You are weird."
else
You can escape content puts "OK."
end
for use in a regex
CONFIG_RE = /A([^=s]+)s*=s*(S+)/
config = "url = http://ruby-lang.org"
There’s a MatchData key, value = config.match(CONFIG_RE).captures
object for matches
fields = "1|2 | 3".split(/s*|s*/)
last_word_i = "one two three".rindex(/bw+/)
Many methods can take
five = "Count: 5"[/d+/]
a regex five = "Count: 5"[/Count:s*(d+)/, 1]

121. Quiz

122. Which of These Does not Match?

123. Which of These Does not Match?
/wreck(ed)?b/i
“wreck” “Wreck”
“wrecked” “wrecks”
“shipwreck” “That’s a wreck.”

124. Which of These Does not Match?
/wreck(ed)?b/i
“wreck” “Wreck”
“wrecked” “wrecks”
“shipwreck” “That’s a wreck.”

125. What Does This Match?

126. What Does This Match?
/A([1-9]|[1-9]d|[12]dd)z/

127. What Does This Match?
/A([1-9]|[1-9]d|[12]dd)z/
A number between 1 and 299.

128. What’s in $1?

129. What’s in $1?
path = "/blogs/3/comments/7/rating.json"
path =~ /(w+)(.w+)?Z/

130. What’s in $1?
path = "/blogs/3/comments/7/rating.json"
path =~ /(w+)(.w+)?Z/
“rating”

131. Advanced Features

132. Regular Expression Extensions

133. Regular Expression Extensions
Ruby’s regex engine
adds several common
extensions

134. Regular Expression Extensions
Ruby’s regex engine
adds several common
extensions
These usually look
something like
(? … )

135. Regular Expression Extensions
Ruby’s regex engine
adds several common
extensions
These usually look
something like
(? … )
The simplest is (?: … )
which is grouping
without capturing

136. Regular Expression Extensions
Ruby’s regex engine
adds several common
data = "put the ball in the sack"
extensions re = %r{
(put|set) # verb: $1
s+ # some space (/x safe)
(?:(?:the|a)s+)? # an article (optional)
These usually look (w+) # noun: $2
s+
something like (?:in(?:side)?)? # preposition (optional)
s+
(? … ) (?:(?:the|a)s+)?
(w+) # noun: $3
}x
p data =~ re
The simplest is (?: … ) p [$1, $2, $3]
which is grouping
without capturing

137. Look-Around Assertions

138. Look-Around Assertions
You can use look-ahead
assertions to peek
ahead without
consuming characters:

139. Look-Around Assertions
You can use look-ahead
assertions to peek
ahead without
consuming characters:
(?= … ) and (?! … )

140. Look-Around Assertions
You can use look-ahead
assertions to peek
ahead without
consuming characters:
(?= … ) and (?! … )
Ruby 1.9 adds a fixed
look-behind:

141. Look-Around Assertions
You can use look-ahead
assertions to peek
ahead without
consuming characters:
(?= … ) and (?! … )
Ruby 1.9 adds a fixed
look-behind:
(?<= … ) and
(?<! … )

142. Look-Around Assertions
You can use look-ahead
assertions to peek
ahead without
consuming characters:
class Numeric
def commify
(?= … ) and (?! … ) to_s.reverse.
gsub(/(ddd)(?=d)(?!d*.)/, '1,').
reverse
end
Ruby 1.9 adds a fixed end
look-behind:
(?<= … ) and
(?<! … )

143. Oniguruma

144. Oniguruma
Ruby 1.9’s regex engine
is faster and more
powerful:

145. Oniguruma
Ruby 1.9’s regex engine
is faster and more
powerful:
Named groups

146. Oniguruma
Ruby 1.9’s regex engine
is faster and more
powerful:
Named groups
Nested matching

147. Oniguruma
Ruby 1.9’s regex engine
is faster and more
powerful:
Named groups
Nested matching
Improved encodings

148. Oniguruma
Ruby 1.9’s regex engine
is faster and more
powerful:
Named groups
Nested matching
Improved encodings
And more…

149. Oniguruma
Ruby 1.9’s regex engine
is faster and more config = "mode = wrap"
if /A(?<key>w+)s*=s*(?<value>w+)/ =~ config
powerful: puts "Key is #{key} and value is #{value}"
end
Named groups
Nested matching
Improved encodings
And more…

150. Oniguruma
Ruby 1.9’s regex engine
is faster and more config = "mode = wrap"
if /A(?<key>w+)s*=s*(?<value>w+)/ =~ config
powerful: puts "Key is #{key} and value is #{value}"
end
Named groups
CHECK = /A(?<paren>((g<paren>|[^()])*?))z/
%w[ ()
(()())
Nested matching (a(b(c,d())))
()) ].each do |test|
unless test =~ CHECK
puts "#{test} isn't balanced"
Improved encodings end
end
And more…

151. A Case Study

152. The Data

153. The Data
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS

154. A First Pass

155. A First Pass
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS
data.scan(/([^,s(][^,(]*?)s*(([^)]+))/) do |name, kind|
p [name, kind]
end
__END__
["Business Name", "Text Field"]
["Allows Pets", "Check"]
["Open To", "Dropdown: Men, Women, Children, Any"]
["Atmosphere", "Check List: Calm, Romantic, New Age"]

156. A First Pass
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS
data.scan(/([^,s(][^,(]*?)s*(([^)]+))/) do |name, kind|
p [name, kind]
end
__END__
["Business Name", "Text Field"]
["Allows Pets", "Check"]
["Open To", "Dropdown: Men, Women, Children, Any"]
["Atmosphere", "Check List: Calm, Romantic, New Age"]

157. A First Pass
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS
data.scan(/([^,s(][^,(]*?)s*(([^)]+))/) do |name, kind|
p [name, kind]
end
__END__
["Business Name", "Text Field"]
["Allows Pets", "Check"]
["Open To", "Dropdown: Men, Women, Children, Any"]
["Atmosphere", "Check List: Calm, Romantic, New Age"]

158. Parsed

159. Parsed
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS
data.scan(/([^,s(][^,(]*?)s*(([^):]+)(?::s*([^)]+?)s*)?)/) do |name, kind, fields|
p [name, kind, fields]
end
__END__
["Business Name", "Text Field", nil]
["Allows Pets", "Check", nil]
["Open To", "Dropdown", "Men, Women, Children, Any"]
["Atmosphere", "Check List", "Calm, Romantic, New Age"]

160. Parsed
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS
data.scan(/([^,s(][^,(]*?)s*(([^):]+)(?::s*([^)]+?)s*)?)/) do |name, kind, fields|
p [name, kind, fields]
end
__END__
["Business Name", "Text Field", nil]
["Allows Pets", "Check", nil]
["Open To", "Dropdown", "Men, Women, Children, Any"]
["Atmosphere", "Check List", "Calm, Romantic, New Age"]

161. Parsed
data = <<END_FIELDS.gsub(/s+/, " ")
Business Name (Text Field),
Allows Pets (Check),
Open To (Dropdown: Men, Women, Children, Any),
Atmosphere (Check List: Calm, Romantic, New Age)
END_FIELDS
data.scan(/([^,s(][^,(]*?)s*(([^):]+)(?::s*([^)]+?)s*)?)/) do |name, kind, fields|
p [name, kind, fields]
end
__END__
["Business Name", "Text Field", nil]
["Allows Pets", "Check", nil]
["Open To", "Dropdown", "Men, Women, Children, Any"]
["Atmosphere", "Check List", "Calm, Romantic, New Age"]