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Project: Ruby-style-guide

Regular Expressions

Some people, when confronted with a problem, think
"I know, I'll use regular expressions." Now they have two problems.

-- Jamie Zawinski

  • Don't use regular expressions if you just need plain text search in string:

  • For simple constructions you can use regexp directly through string index.

    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'
  • Use non-capturing groups when you don't use captured result of parentheses.

    /(first|second)/   # bad
    /(?:first|second)/ # good
  • Don't use the cryptic Perl-legacy variables denoting last regexp group matches
    ($1, $2, etc). Use Regexp.last_match[n] instead.

    /(regexp)/ =~ string
    # bad
    process $1
    # good
    process Regexp.last_match[1]
  • Avoid using numbered groups as it can be hard to track what they contain. Named groups
    can be used instead.

    # bad
    /(regexp)/ =~ string
    process Regexp.last_match[1]
    # good
    /(?<meaningful_var>regexp)/ =~ string
    process meaningful_var
  • Character classes have only a few special characters you should care about:
    ^, -, \, ], so don't escape . or brackets in [].

  • Be careful with ^ and $ as they match start/end of line, not string endings.
    If you want to match the whole string use: \A and \z (not to be
    confused with \Z which is the equivalent of /\n?\z/).

    string = "some injection\nusername"
    string[/^username$/]   # matches
    string[/\Ausername\z/] # doesn't match
  • Use x modifier for complex regexps. This makes them more readable and you
    can add some useful comments. Just be careful as spaces are ignored.

    regexp = /
      start         # some text
      \s            # white space char
      (group)       # first group
      (?:alt1|alt2) # some alternation
  • For complex replacements sub/gsub can be used with block or hash.

Last published almost 7 years ago by David Kariuki.