How to match bar, b-a-r, b--a--r etc in a string by Regexp - ruby

Question

Given a string, I want to find a word bar, b-a-r, b--a--r etc. where - can be any letter. But interval between letters must be the same.
All letters are lower case and there is no gap betweens.
For example bar, beayr, qbowarprr, wbxxxxxayyyyyrzzz should match this.
I tried /b[a-z]*a[a-z]*r/ but this matches bxar which is wrong.
I am wondering if I achieve this with regexp?

Answer 1

Here's is one way to get all matches.
Code
def all_matches_with_spacers(word, str)
word_size = word.size
word_arr = word.chars
str_arr = str.chars
(0..(str.size - word_size)/(word_size-1)).each_with_object([]) do |n, arr|
regex = Regexp.new(word_arr.join(".{#{n}}"))
str_arr.each_cons(word_size + n * (word_size - 1))
.map(&:join)
.each { |substring| arr << substring if substring =~ regex }
end
end
This requires word.size > 1.
Example
all_matches_with_spacers('bar', 'bar') #=> ["bar"]
all_matches_with_spacers('bar', 'beayr') #=> ["beayr"]
all_matches_with_spacers('bar', 'qbowarprr') #=> ["bowarpr"]
all_matches_with_spacers('bar', 'wbxxxxxayyyyyrzzz') #=> ["bxxxxxayyyyyr"]
all_matches_with_spacers('bobo', 'bobobocbcbocbcobcodbddoddbddobddoddbddob')
#=> ["bobo", "bobo", "bddoddbddo", "bddoddbddo"]
Explanation
Suppose
word = 'bobo'
str = 'bobobocbcbocbcobcodbddoddbddobddoddbddob'
then
word_size = word.size #=> 4
word_arr = word.chars #=> ["b", "o", "b", "o"]
str_arr = str.chars
#=> ["b", "o", "b", "o", "b", "o", "c", "b", "c", "b", "o", "c", "b", "c",
# "o", "b", "c", "o", "d", "b", "d", "d", "o", "d", "d", "b", "d", "d",
# "o", "b", "d", "d", "o", "d", "d", "b", "d", "d", "o", "b"]
If n is the number of spacers between each letter of word, we require
word.size + n * (word.size - 1) <= str.size
Hence (since str.size => 40),
n <= (str.size - word_size)/(word_size-1) #=> (40-4)/(4-1) => 12
We therefore will iterate over zero to 12 spacers:
(0..12).each_with_object([]) do |n, arr| .. end
Enumerable#each_with_object creates an initially-empty array denoted by the block variable arr. The first value passed to block is zero (spacers), assigned to the block variable n.
We then have
regex = Regexp.new(word_arr.join(".{#{0}}")) #=> /b.{0}o.{0}b.{0}o/
which is the same as /bar/. word with n spacers has length
word_size + n * (word_size - 1) #=> 19
To extract all sub-arrays of str_arr with this length, we invoke:
str_arr.each_cons(word_size + n * (word_size - 1))
Here, with n = 0, this is:
enum = str_arr.each_cons(4)
#=> #<Enumerator: ["b", "o", "b", "o", "b", "o",...,"b"]:each_cons(4)>
This enumerator will pass the following into its block:
enum.to_a
#=> [["b", "o", "b", "o"], ["o", "b", "o", "b"], ["b", "o", "b", "o"],
# ["o", "b", "o", "c"], ["b", "o", "c", "b"], ["o", "c", "b", "c"],
# ["c", "b", "c", "b"], ["b", "c", "b", "o"], ["c", "b", "o", "c"],
# ["b", "o", "c", "b"], ["o", "c", "b", "c"], ["c", "b", "c", "o"],
# ["b", "c", "o", "b"], ["c", "o", "b", "c"], ["o", "b", "c", "o"]]
We next convert these to strings:
ar = enum.map(&:join)
#=> ["bobo", "obob", "bobo", "oboc", "bocb", "ocbc", "cbcb", "bcbo",
# "cboc", "bocb", "ocbc", "cbco", "bcob", "cobc", "obco"]
and add each (assigned to the block variable substring) to the array arr for which:
substring =~ regex
ar.each { |substring| arr << substring if substring =~ regex }
arr => ["bobo", "bobo"]
Next we increment the number of spacers to n = 1. This has the following effect:
regex = Regexp.new(word_arr.join(".{#{1}}")) #=> /b.{1}o.{1}b.{1}o/
str_arr.each_cons(4 + 1 * (4 - 1)) #=> str_arr.each_cons(7)
so we now examine the strings
ar = str_arr.each_cons(7).map(&:join)
#=> ["boboboc", "obobocb", "bobocbc", "obocbcb", "bocbcbo", "ocbcboc",
# "cbcbocb", "bcbocbc", "cbocbco", "bocbcob", "ocbcobc", "cbcobco",
# "bcobcod", "cobcodb", "obcodbd", "bcodbdd", "codbddo", "odbddod",
# "dbddodd", "bddoddb", "ddoddbd", "doddbdd", "oddbddo", "ddbddob",
# "dbddobd", "bddobdd", "ddobddo", "dobddod", "obddodd", "bddoddb",
# "ddoddbd", "doddbdd", "oddbddo", "ddbddob"]
ar.each { |substring| arr << substring if substring =~ regex }
There are no matches with one spacer, so arr remains unchanged:
arr #=> ["bobo", "bobo"]
For n = 2 spacers:
regex = Regexp.new(word_arr.join(".{#{2}}")) #=> /b.{2}o.{2}b.{2}o/
str_arr.each_cons(4 + 2 * (4 - 1)) #=> str_arr.each_cons(10)
ar = str_arr.each_cons(10).map(&:join)
#=> ["bobobocbcb", "obobocbcbo", "bobocbcboc", "obocbcbocb", "bocbcbocbc",
# "ocbcbocbco", "cbcbocbcob", "bcbocbcobc", "cbocbcobco", "bocbcobcod",
# ...
# "ddoddbddob"]
ar.each { |substring| arr << substring if substring =~ regex }
arr #=> ["bobo", "bobo", "bddoddbddo", "bddoddbddo"]
No matches are found for more than two spacers, so the method returns
["bobo", "bobo", "bddoddbddo", "bddoddbddo"]

Answer 2

For reference, there is a beautiful solution to the overall problem that is available in regex flavors that allow a capturing group to refer to itself:
^[^b]*bar|b(?:[^a](?=[^a]*a(\1?+.)))+a\1r
Sadly, Ruby doesn't allow this.
The interesting bit is on the right side of the alternation. After matching the initial b, we define a non-capturing group for the characters between b and a. This group will be repeated with the +. Between the a and r, we will inject capture group 1 with \1`. This group was captured one character at a time, overwriting itself with each pass, as each character between b and a was added.
See Quantifier Capture where the solution was demonstrated by #CasimiretHippolyte who refers to the idea behind the technique the "qtax trick".