From: Carlos Ortega <caof2005@...>
Date: 2007-12-31T15:25:56+09:00
Subject: Questions about * + and ? in Regex

Hi,

I have some questions related the correct meaning of * + and ? in Regex
that I would appreciate some clarification:
I have  an example (derived from the Programming Ruby 2nd Edition), that
I don't understand why gives these results, here is the code:

def show_regexp(a, re)
  if a =~ re
    puts "#{$`}<<#{$&}>>#{$'}"
  else
    puts "no match"
  end
end

show_regexp('Example1', /\s*/)
show_regexp('Example2', /\s.*/)
show_regexp('Example3 ', /\s.?/)        # Space at the end of string
show_regexp('Example4 ', /\s.+/)       # Space at the end of string
show_regexp('Example5 ', /\s.*/)       # Space at the end of string

output gives:

<<>>Example1
no match
Example3<< >>
no match
Example5<< >>

If I understand well:
     * means - match zero or more occurrences of  preceding expression.
     + means - match 1 or more occurrences of  preceding expression.
     ?  means - match 0 or 1 occurrence of  preceding expression.

Why Example2 gives "no match"? I understand this as find "0 or more
occurrences" of (a space  followed by any character)
Why Example4 gives "no match"? I understand this as find "1 or more
occurrence" of (a space  followed by any character)
I am assuming that the null character can be matched by a .
Am I correct?

Best Regards
-- 
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