by Benjohn Barnes
Usually, you write a regular expression and then search for it in a text string.
How about doing the opposite? Given a regular expression, generate all of the
strings that it will match.
Hi, this is my solution. For the design I took the following decisions:
- I will limit the number of repetitions provided by * and + to a
constant. In my examples I've set it to 2
- I will limit the set of characters generated by . and [^...]
constructs to a fixed list of chars. For my tests I used CHARS = %w{a
b c d e}, but also tested with
#CHARS = [("a".."z").to_a, ("A".."Z").to_a, ".", ",", ";"].flatten
but the results can be big !!
I have the concept of repeaters and groups. A repeteater is a class
that knows how to repeat the result of the groups it contains. I've
implemented OneTimeRepeater for the groups without repetition
modifier, StarRepeater, PlusRepeater and QuestionMarkRepeater for the
appropriate modifiers. RangeRepeater takes care of the {a,b} modifier.
Groups: a group represents a piece of the string for which we can
geneate a set of possible values. The groups I have implemented are
SingleChar, CharGroup (don't support ranges yet, just a list of chars)
and Dot. I haven't implemented escaped characters like \s, \d, etc,
but I think they are trivial to implement. I have a couple of special
groups to cope with parens and |. MultiGroup takes care of the parens
nesting by combining the result of the contained repeaters, and
OrGroup adds together the result of the contained groups.
Things I don't support:
- Ranges in char groups [a-zA-Z]
- non-greedy repetitions: I don't even know how to do this, cause I
think you have to take into account what you are going to generate
after the group
- Character classes: \s, \d, etc and [:alpha:], etc, but I think they
are easy to implement
- Backreferences: /(abc)\1/, didn't even thought about them, but
probably a nightmare.
- Other things I don't even know
.
Most of the interesting stuff is in the parse and combine methods. The
first one understands the syntax and creates groups and repeaters.
Uses recursive calls for parens and |. The combine method is able to
combine an array of arrays to produce all the possible combinations:
[["", "a"], ["b", "bb"]] --> ["b", "bb", "ab", "abb"]
Here it is:
# Number of times to repeat for Star and Plus repeaters
TIMES = 2
# Set of chars for Dot and negated [^] char groups
#CHARS = [("a".."z").to_a, ("A".."Z").to_a, ".", ",", ";"].flatten
CHARS = %w{a b c d e}
class OneTimeRepeater
def initialize(group)
@group = group
end
def result
@group.result
end
end
class StarRepeater
def initialize(group)
@group = group
end
def result
r = []
group_res = @group.result
group_res.unshift("")
TIMES.times do
r << group_res
end
combine(r).uniq
end
end
class PlusRepeater
def initialize(group)
@group = group
end
def result
group_res = @group.result
r = [group_res]
temp = [""].concat(group_res)
(TIMES - 1).times do
r << temp
end
combine(r).uniq
end
end
class QuestionMarkRepeater
def initialize(group)
@group = group
end
def result
@group.result.unshift("")
end
end
class RangeRepeater
def initialize(group,min,max)
@group = group
@min = min
@max = max
end
def result
result = @group.result
r = []
r << [""] if @min == 0
@min.times {r << result}
temp = result.dup.unshift("")
(
@max - @min).times do
r << temp
end
combine(r).uniq
end
end
class SingleChar
def initialize(c)
@c = c
end
def result
[@c]
end
end
# TODO: support ranges [a-zA-Z]
class CharGroup
def initialize(chars)
@negative = chars[0] == "^"
@chars = chars
@chars = @chars[1..-1] if @negative
end
def result
if @negative
CHARS - @chars
else
@chars
end
end
end
class Dot
def result
CHARS
end
end
class MultiGroup
def initialize(groups)
@groups = groups
end
def result
strings = @groups.map {|x| x.result}
combine(strings)
end
end
class OrGroup
def initialize(first_groupset, second_groupset)
@first = first_groupset
@second = second_groupset
end
def result
strings = @first.map {|x| x.result}
s = combine(strings)
strings = @second.map {|x| x.result}
s.concat(combine(strings))
end
end
# Combines arrays, calling + on each possible pair
# Starts from the first two arrays, then goes on
# combining another array to the result
def combine(arrays)
string = arrays.inject do |r, rep|
temp = []
r.each {|aa| rep.each {|bb| temp << (aa + bb)}}
temp
end
string
end
def parse(s, i = 0)
repeaters = []
group = nil
while i < s.length
char = s
.chr
case char
when '('
groups, i = parse(s, i+1)
group = MultiGroup.new(groups)
when ')'
return repeaters,i
when '['
chars = []
i += 1
until s.chr == ']'
chars << s.chr
i += 1
end
group = CharGroup.new(chars)
when '.'
group = Dot.new
when '|'
groups, i = parse(s, i + 1)
group = OrGroup.new(repeaters, groups)
return [group], i
else
group = SingleChar.new(char)
end
repeater = nil
i += 1
if i < s.length
case s.chr
when '*'
repeater = StarRepeater.new(group)
when '+'
repeater = PlusRepeater.new(group)
when '?'
repeater = QuestionMarkRepeater.new(group)
when '{'
first = ""
i += 1
while s.chr != ","
first << s.chr
i += 1
end
i += 1
second = ""
while s.chr != "}"
second << s.chr
i += 1
end
repeater = RangeRepeater.new(group, first.to_i, second.to_i)
else
repeater = OneTimeRepeater.new(group)
i -= 1
end
i += 1
else
repeater = OneTimeRepeater.new(group)
end
repeaters << repeater
end
return repeaters, i
end
class Regexp
def generate
r = self.inspect[1..-2]
repeaters, _ = parse(r)
strings = repeaters.map {|x| x.result}
s = combine(strings)
s
end
end
def show(regexp)
s = regexp.generate
puts "#{regexp.inspect} --> #{s.inspect}"
#puts "Checking..."
#errors = s.reject {|string| string =~ regexp}
#if errors.size == 0
# puts "All strings match"
#else
# puts "These don't match: #{errors.inspect}"
#end
#
end
Some tests with TIMES=2 and CHARS = %w{a b c d e}:
show(/ab+[def]?[ghi]j/)
show(/ab*c+/)
show(/ab(c+(de)*)?f/)
show(/a{0,3}/)
show(/a|b|c/)
show(/ab(c)+|xy*|jjjk+[^jk]/)
show(/(lovely|delicious|splendid)(food|snacks|munchies)/)
/ab+[def]?[ghi]j/ --> ["abgj", "abhj", "abij", "abdgj", "abdhj",
"abdij", "abegj", "abehj", "abeij", "abfgj", "abfhj", "abfij",
"abbgj", "abbhj", "abbij", "abbdgj", "abbdhj", "abbdij", "abbegj",
"abbehj", "abbeij", "abbfgj", "abbfhj", "abbfij"]
/ab*c+/ --> ["ac", "acc", "abc", "abcc", "abbc", "abbcc"]
/ab(c+(de)*)?f/ --> ["abf", "abcf", "abcdef", "abcdedef", "abccf",
"abccdef", "abccdedef"]
/a{0,3}/ --> ["", "a", "aa", "aaa"]
/a|b|c/ --> ["a", "b", "c"]
/ab(c)+|xy*|jjjk+[^jk]/ --> ["abc", "abcc", "x", "xy", "xyy", "jjjka",
"jjjkb", "jjjkc", "jjjkd", "jjjke", "jjjkka", "jjjkkb", "jjjkkc",
"jjjkkd", "jjjkke"]
/(lovely|delicious|splendid)(food|snacks|munchies)/ --> ["lovelyfood",
"lovelysnacks", "lovelymunchies", "deliciousfood", "delicioussnacks",
"deliciousmunchies", "splendidfood", "splendidsnacks",
"splendidmunchies"]
It was fun.
Regards,
Jesus.