The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

3.5. Using Quantifiers

A big part of regular expressions is handling optional items and repetition. An item followed by a question mark is optional; it may be present or absent, and the match depends on the rest of the regex. (It doesn’t make sense to apply this to an anchor but only to a subpattern of non-zero width.)

pattern = /ax?b/
pat2    = /a[xy]?b/
pattern =~ "ab"     # 0
pattern =~ "acb"    # nil
pattern =~ "axb"    # 0
pat2    =~ "ayb"    # 0
pat2    =~ "acb"    # nil

It is common for entities to be repeated an indefinite number of times (which we can specify with the + quantifier). For example, this pattern matches any positive integer:

pattern = /[0-9]+/
pattern =~ "1"         # 0
pattern =~ "2345678"   # 0

Another common occurrence is a pattern that occurs zero or ...