match = string[/regexp/]       # get content of matched regexp
  first_group = string[/text(grp)/, 1] # get content of captured group
  string[/text (grp)/, 1] = 'replace' # string => 'text replace'

  /(first|second)/  # bad
  /(?:first|second)/ # good

  /(regexp)/ =~ string
  ...

  # bad
  process $1

  # good
  process Regexp.last_match[1]

  # bad
  /(regexp)/ =~ string
  ...
  process Regexp.last_match[1]

  # good
  /(?<meaningful_var>regexp)/ =~ string
  ...
  process meaningful_var

  string = "some injectionnusername"
  string[/^username$/]  # matches
  string[/AusernameZ/] # don't match

  regexp = %r{
   start     # some text
   s      # white space char
   (group)    # first group
   (?:alt1|alt2) # some alternation
   end
  }x