Regular Expression in Ruby

In this post, I intend to write down some basic Regex knowledge, especially for Ruby.

Syntax

A regular expression literal is a pattern between slashes or between arbitrary delimiters followed by %r as follows:

  /pattern/
  /pattern/im    # option can be specified
  %r!/usr/local! # general delimited regular expression

Basic patterns

Except for control characters, (+ ? . * ^ $ ( ) [ ] { } | \ ), all characters match themselves. You can escape a control character by preceding it with a backslash.

Following table lists the regular expression syntax that is available in Ruby.

Patterns	Description
^	Matches beginning of line.
$	Matches end of line.
.	Matches any single character except newline. Using m option to allow it to match newline as well.
[…]	Matches any single character in brackets.
[^…]	Matches any single character not in brackets
re*	Matches 0 or more occurrences of preceding expression.
re+	Matches 1 or more occurrence of preceding expression.
re?	Matches 0 or 1 occurrence of preceding expression.
re{n}	Matches exactly n number of occurrences of preceding expression.
re{n,}	Matches n or more occurrences of preceding expression.
re{n, m}	Matches at least n and at most m occurrences of preceding expression.
a|b	Matches either a or b.
(re)	Groups regular expressions and remembers matched text.
(?imx)	Temporarily toggles on i, m, or x options within a regular expression. If in parentheses, only that area is affected.
(?-imx)	Temporarily toggles off i, m, or x options within a regular expression. If in parentheses, only that area is affected.
(?: re)	Groups regular expressions without remembering matched text.
(?imx: re)	Temporarily toggles on i, m, or x options within parentheses.
(?-imx: re)	Temporarily toggles off i, m, or x options within parentheses.
(?#…)	Comment.
(?= re)	Specifies position using a pattern. Doesn’t have a range.
(?! re)	Specifies position using pattern negation. Doesn’t have a range.
(?> re)	Matches independent pattern without backtracking.
\w	Matches word characters.
\W	Matches nonword characters.
\s	Matches whitespace. Equivalent to [\t\n\r\f].
\S	Matches nonwhitespace.
\d	Matches digits. Equivalent to [0-9].
\D	Matches nondigits.
\A	Matches beginning of string.
\Z	Matches end of string. If a newline exists, it matches just before newline.
\z	Matches end of string.
\G	Matches point where last match finished.
\b	Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets.
\B	Matches nonword boundaries.
\n, \t, etc.	Matches newlines, carriage returns, tabs, etc.
\1…\9	Matches nth grouped subexpression.
\10	Matches nth grouped subexpression if it matched already. Otherwise refers to the octal representation of a character code.

Examples

Examples	Description
/[Rr]uby/	Match “Ruby” or “ruby”
/rub[ye]/	Match “ruby” or “rube”
/[aeiou]/	Match any one lowercase vowel
/[0-9]/	Match any digit; same as /[0123456789]/
/[a-z]/	Match any lowercase ASCII letter
/[A-Z]/	Match any uppercase ASCII letter
/[a-zA-Z0-9]/	Match any of the above
/[^aeiou]/	Match anything other than a lowercase vowel
/[^0-9]/	Match anything other than a digit

Searching and replace in ruby

"0123-123-123 is a phone number.".gsub(/[\D]/, "") # 0123123123
line1 = "Cats are smarter than dogs"
line2 = "Dogs also like meat"
line1 =~ /Cats(.*)/ # true
line2 =~ /Cats(.*)/ #false

References: https://www.tutorialspoint.com/ruby/ruby_regular_expressions.htm