[ruby-core:112186] [Ruby master Bug#19402] CSV skip_lines option not behaving as documented

Issue #19402 has been updated by sawa (Tsuyoshi Sawada).


I agree with you that the description in the documentation is bad, but for a reason different from what you claim. The problem is that it is ambiguous. It says that the string is converted to a Regexp, but it does not specify how. That leaves a room for the reader to interpret it in one or another way.

Perhaps, you interpreted that a string `str` passed as the `skip_lines:` argument is converted to a Regexp by:

```ruby
Regexp.new(str)
```

However, I believe the intended interpretation was to convert the string argument to a Regexp by:

```ruby
Regexp.new(Regexp.escape(str))
```

in which case matching against the resulting Regexp is equivalent to a substring check against the original string, and the results you got is just as described in the documentation.

I agree with you that the documentation can be improved.

----------------------------------------
Bug #19402: CSV skip_lines option not behaving as documented
https://bugs.ruby-lang.org/issues/19402#change-101623

* Author: jamie_ca (Jamie Macey)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.0 (2022-12-25 revision a528908271) [x86_64-darwin21]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
The [CSV documentation](https://ruby-doc.org/stdlib-3.1.0/libdoc/csv/rdoc/CSV.html#class-CSV-label-Option+skip_lines) for the `skip_lines` parser option says "If a String, converts it to a Regexp, ignores lines that match it." Application behaviour as well as [the source](https://github.com/ruby/csv/blob/master/lib/csv/parser.rb#L909-L919) appears to be normalizing the string encoding and running a simple substring check instead. Given the existing behaviour, this might just want a documentation update to describe it accurately?

I stumbled across this on a project still on ruby 2.6.9 ([2.6 docs](https://ruby-doc.org/stdlib-2.6.1/libdoc/csv/rdoc/CSV.html#method-c-new)), but it's applicable still at 3.2.0.

Reproduction script:

```ruby
require 'csv'

data = <<CSV
data,data
test,data
data,test
CSV

puts "Parsing with regexp skip_lines /^test/, expect 2 rows"
CSV.parse(data, skip_lines: /^test/).each { |row| pp row }
puts

puts "Parsing with text skip_lines \"^test\", expect 2 rows"
CSV.parse(data, skip_lines: "^test").each { |row| pp row }
puts

puts "Parsing with unanchored text skip_lines \"test\", expect 1 row"
CSV.parse(data, skip_lines: "test").each { |row| pp row }
puts
```

```
$ ruby csv_test.rb
Parsing with regexp skip_lines /^test/, expect 2 rows
["data", "data"]
["data", "test"]

Parsing with text skip_lines "^test", expect 2 rows
["data", "data"]
["test", "data"]
["data", "test"]

Parsing with unanchored text skip_lines "test", expect 1 row
["data", "data"]
```




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