Hi!
I couldn''t understand the behavior of this code:
match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)?/)
a = match.to_a
puts a.size # 2
puts a.join(",") # Feb 23rd,rd
puts a[0] # Feb 23rd
puts a[1] # rd
In my understanding, /Feb 23(rd)?/ is equivalent to /Feb 23|Feb
23rd/ . So, match should not include ''rd''.
Thanks.
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On 19 Oct 2008, at 15:01, mars wrote:> > Hi! > > I couldn''t understand the behavior of this code: > > match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)?/) > a = match.to_a > puts a.size # 2 > puts a.join(",") # Feb 23rd,rd > puts a[0] # Feb 23rd > puts a[1] # rd > > In my understanding, /Feb 23(rd)?/ is equivalent to /Feb 23|Feb > 23rd/ . So, match should not include ''rd''.? + and * are greedy, ie they always try to match as much of the string as possible so rd is part of the match. If you want a non greedy quantifier you need to add ? to it, for example match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)??/) match[0] #=> "Feb 23" Fred --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org To unsubscribe from this group, send email to rubyonrails-talk+unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---
Yeah you''re right,> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)??/) > match[0] #=> "Feb 23" > match[1] #=> nilI expected:> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)?/) # ? is greedy here > match[0] #=> "Feb 23rd" > match[1] #=> nilBut what I got in ruby 1.8.6 (2007-09-24 patchlevel 111) [i386-mswin32] is:> match[0] #=> "Feb 23rd" > match[1] #=> ''rd''The behavior of ''?'' being greedy is correct since it matched "Feb 23rd" which is stored in match[0] . But should match[1] not be nil? The regular expression does not match "rd" alone. thanks On Sun, Oct 19, 2008 at 11:34 PM, Frederick Cheung <frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:> > > On 19 Oct 2008, at 15:01, mars wrote: > >> >> Hi! >> >> I couldn''t understand the behavior of this code: >> >> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)?/) >> a = match.to_a >> puts a.size # 2 >> puts a.join(",") # Feb 23rd,rd >> puts a[0] # Feb 23rd >> puts a[1] # rd >> >> In my understanding, /Feb 23(rd)?/ is equivalent to /Feb 23|Feb >> 23rd/ . So, match should not include ''rd''. > > ? + and * are greedy, ie they always try to match as much of the > string as possible so rd is part of the match. > If you want a non greedy quantifier you need to add ? to it, for example > match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)??/) > match[0] #=> "Feb 23" > > Fred > > > >--~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org To unsubscribe from this group, send email to rubyonrails-talk+unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---
On 19 Oct 2008, at 16:04, Marcelino Debajo wrote:> > Yeah you''re right, > >> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)??/) >> match[0] #=> "Feb 23" >> match[1] #=> nil > > I expected: > >> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)?/) # ? is >> greedy here >> match[0] #=> "Feb 23rd" >> match[1] #=> nil > > But what I got in ruby 1.8.6 (2007-09-24 patchlevel 111) [i386- > mswin32] is: >> match[0] #=> "Feb 23rd" >> match[1] #=> ''rd'' > > The behavior of ''?'' being greedy is correct since it matched "Feb > 23rd" which is stored in match[0] . But should match[1] not be nil? > The regular expression does not match "rd" alone.match[1] is the first group, which in this example should be ''rd'' (unless you''re talking about something else). Fred --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org To unsubscribe from this group, send email to rubyonrails-talk+unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---
thanks Fred. I think I misunderstood something. On Mon, Oct 20, 2008 at 12:14 AM, Frederick Cheung <frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:> > > On 19 Oct 2008, at 16:04, Marcelino Debajo wrote: > >> >> Yeah you''re right, >> >>> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)??/) >>> match[0] #=> "Feb 23" >>> match[1] #=> nil >> >> I expected: >> >>> match = ''Today is Feb 23rd, 2003''.match(/Feb 23(rd)?/) # ? is >>> greedy here >>> match[0] #=> "Feb 23rd" >>> match[1] #=> nil >> >> But what I got in ruby 1.8.6 (2007-09-24 patchlevel 111) [i386- >> mswin32] is: >>> match[0] #=> "Feb 23rd" >>> match[1] #=> ''rd'' >> >> The behavior of ''?'' being greedy is correct since it matched "Feb >> 23rd" which is stored in match[0] . But should match[1] not be nil? >> The regular expression does not match "rd" alone. > > > match[1] is the first group, which in this example should be > ''rd'' (unless you''re talking about something else). > > Fred > > > >--~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org To unsubscribe from this group, send email to rubyonrails-talk+unsubscribe-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---