Ruby regex to get text blocks including delimiters - ruby

Question

When using scan in Ruby, we are searching for a block within a text file.
Sample file:
sometextbefore
begin
sometext
end
sometextafter
begin
sometext2
end
sometextafter2
We want the following result in an array:
["begin\nsometext\nend","begin\nsometext2\nend"]
With this scan method:
textfile.scan(/begin\s.(.*?)end/m)
we get:
["sometext","sometext2"]
We want the begin and end still in the output, not cut off.
Any suggestions?

Answer 1

You may remove the capturing group completely:
textfile.scan(/begin\s.*?end/m)
See the IDEONE demo
The String#scan method returns captured values only if you have capturing groups defined inside the pattern, thus a non-capturing one should fix the issue.
UPDATE
If the lines inside the blocks must be trimmed from leading/trailing whitespace, you can just use a gsub against each matched block of text to remove all the horizontal whitespace (with the help of \p{Zs} Unicode category/property class):
.scan(/begin\s.*?end/m).map { |s| s.gsub(/^\p{Zs}+|\p{Zs}+$/, "") }
Here, each match is passed to a block where /^\p{Zs}+|\p{Zs}+$/ matches either the start of a line with 1+ horizontal whitespace(s) (see ^\p{Zs}+), or 1+ horizontal whitespace(s) at the end of the line (see \p{Zs}+$).
See another IDEONE demo

Answer 2

Here's another approach, using Ruby's flip-flop operator. I cannot say I would recommend this approach, but Rubiests should understand how the flip-flop operator works.
First let's create a file.
str =<<_
some
text
at beginning
begin
some
text
1
end
some text
between
begin
some
text
2
end
some text at end
_
#=> "some\ntext\nat beginning\nbegin\n some\n text\n 1\nend\n...at end\n"
FName = "text"
File.write(FName, str)
Now read the file line-by-line into the array lines:
lines = File.readlines(FName)
#=> ["some\n", "text\n", "at beginning\n", "begin\n", " some\n", " text\n",
# " 1\n", "end\n", "some text\n", "between\n", "begin\n", " some\n",
# " text\n", " 2\n", "end\n", "some text at end\n"]
We can obtain the desired result as follows.
lines.chunk { |line| true if line =~ /^begin\s*$/ .. line =~ /^end\s*$/ }.
map { |_,arr| arr.map(&:strip).join("\n") }
#=> ["begin\nsome\ntext\n1\nend", "begin\nsome\ntext\n2\nend"]
The two steps are as follows.
First, select and group the lines of interest, using Enumerable#chunk with the flip-flop operator.
a = lines.chunk { |line| true if line =~ /^begin\s*$/ .. line =~ /^end\s*$/ }
#=> #<Enumerator: #<Enumerator::Generator:0x007ff62b981510>:each>
We can see the objects that will be generated by this enumerator by converting it to an array.
a.to_a
#=> [[true, ["begin\n", " some\n", " text\n", " 1\n", "end\n"]],
# [true, ["begin\n", " some\n", " text\n", " 2\n", "end\n"]]]
Note that the flip-flop operator is distinguished from a range definition by making it part of a logical expression. For that reason we cannot write
lines.chunk { |line| line =~ /^begin\s*$/ .. line =~ /^end\s*$/ }.to_a
#=> ArgumentError: bad value for range
The second step is the following:
b = a.map { |_,arr| arr.map(&:strip).join("\n") }
#=> ["begin\nsome\ntext\n1\nend", "begin\nsome\ntext\n2\nend"]

Answer 3

Ruby has some great methods in Enumerable. slice_before and slice_after can help with this sort of problem:
string = <<EOT
sometextbefore
begin
sometext
end
sometextafter
begin
sometext2
end
sometextafter2
EOT
ary = string.split # => ["sometextbefore", "begin", "sometext", "end", "sometextafter", "begin", "sometext2", "end", "sometextafter2"]
.slice_after(/^end/) # => #<Enumerator: #<Enumerator::Generator:0x007fb1e20b42a8>:each>
.map{ |a| a.shift; a } # => [["begin", "sometext", "end"], ["begin", "sometext2", "end"], []]
ary.pop # => []
ary # => [["begin", "sometext", "end"], ["begin", "sometext2", "end"]]
If you want the resulting sub-arrays joined then that's an easy step:
ary.map{ |a| a.join("\n") } # => ["begin\nsometext\nend", "begin\nsometext2\nend"]