From: Chris Shea <chris@...>
Date: 2007-12-31T15:50:27+09:00
Subject: Re: Questions about * + and ? in Regex

On Dec 30, 2007 11:25 PM, Carlos Ortega <caof2005@yahoo.com> wrote:
> 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
> --
> Posted via http://www.ruby-forum.com/.
>
>

A dot (.) can only match an actual character. Example 2 fails because
it's looking not for "0 or more occurrences of (a space  followed by
any character)", but "a space followed by 0 or more characters". The *
only applies to whatever immediately precedes it, not the whole
expression... unless the expression's enclosed in parentheses. A regex
for "0 or more occurrences of (a space  followed by any character)"
would be /(\s.)*/. In that case, the * applies to the parenthesized
group of whitespace and dot.

Example 4 fails because the only space isn't followed by anything at all.

HTH,
Chris

P.S. I strongly recommend Jeffrey Friedl's Mastering Regular Expressions.