Named captures not supported in `StringScanner` on Ruby < 2.1

[tomeon]

Support for named captures in StringScanner was introduced in Ruby 2.1 on 2013-05-21; see the changelog entry.

There are several locations in which Kramdown::Utils::StringScanner#[] is invoked using a non-integer argument -- for example, line 48 of kramdown/parser/kramdown/header.rb. On Ruby 2.0.0, this can lead to errors like:

TypeError: no implicit conversion of String into Integer
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown/header.rb:48:in `[]'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown/header.rb:48:in `parse_header_contents'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown/header.rb:34:in `parse_atx_header'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:148:in `block (2 levels) in parse_blocks'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:146:in `each'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:146:in `any?'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:146:in `block in parse_blocks'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:144:in `catch'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:144:in `parse_blocks'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/kramdown.rb:89:in `parse'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/parser/base.rb:69:in `parse'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-1.17.0/lib/kramdown/document.rb:104:in `initialize'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-man-0.1.6/lib/kramdown/man/task.rb:62:in `new'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-man-0.1.6/lib/kramdown/man/task.rb:62:in `render'
/rbexec/vendor/bundle/ruby/2.0.0/gems/kramdown-man-0.1.6/lib/kramdown/man/task.rb:47:in `block (2 levels) in define'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/cli/exec.rb:74:in `load'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/cli/exec.rb:74:in `kernel_load'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/cli/exec.rb:28:in `run'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/cli.rb:424:in `exec'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/vendor/thor/lib/thor/invocation.rb:126:in `invoke_command'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/vendor/thor/lib/thor.rb:387:in `dispatch'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/cli.rb:27:in `dispatch'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/vendor/thor/lib/thor/base.rb:466:in `start'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/cli.rb:18:in `start'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/exe/bundle:30:in `block in <top (required)>'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/lib/bundler/friendly_errors.rb:124:in `with_friendly_errors'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/gems/bundler-1.16.3/exe/bundle:22:in `<top (required)>'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/bin/bundle:23:in `load'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/bin/bundle:23:in `<main>'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/bin/ruby_executable_hooks:24:in `eval'
/home/travis/.rvm/gems/ruby-2.0.0-p648@global/bin/ruby_executable_hooks:24:in `<main>'

I believe that the simplest solution would be to deprecate support for Ruby < 2.1 via setting s.required_ruby_version = '>= 2.1' in the Rakefile, and I'd be happy to submit a PR introducing this change. I would also be happy to investigate a 2.0-compatible solution if dropping support for Ruby < 2.1 is not acceptable at this time.

[UltimateGeek]

I think this started happening in 1.17.0 of kramdown. When I use kramdown 1.16.2 on ruby2.0, I don't see this problem. I agree with @tomeon that the gemspec needs to be updated to indicate a minimum requirement on ruby 2.1.

[gettalong]

Thanks for the report! I think deprecating support for deprecated Ruby versions will be the way to go.

[gettalong]

The next version of kramdown will require at least Ruby 2.3 since that is the lowest Ruby version still supported.

[tomeon]

TYVM, @gettalong!