From: 7stud -- <bbxx789_05ss@...>
Date: 2008-04-07T13:09:43+09:00
Subject: Re: Regex and non-greedy matching?

Marc Heiler wrote:
> I have a slight problem. I have strings with some tags such as
> 
> '<b><lightblue>name:</></b>'
> 
> I need to match "name:" and "lightblue"
> In other words:
>   - What is between <> </>
> and
>   - What is inside the first <> right next to "name:"
> 
> The following regex does not work:
> 
> '<b><lightblue>name:</></b>' =~ /<([a-zA-Z]+)>(.+?)<\/>/
> 
>   $1  # => "b"
>

This is your string:

'<b><lightblue>name:</></b>'

and the first part of your regex says to look for a '<', followed by one 
or more characters, followed by a '>'.  That certainly describes the 
string '<b>'.


>   $2  # => "<lightblue>name:

This is your string again:

'<b>  <--already matched this
    <lightblue>name:</></b>'

The second part of your regex says to look for a '<', followed by any 
character one or more times, followed by '</>'.  That certainly 
describes the string '<lightblue>name</>'.

Note that since the characters '</>' only appear once in your string, 
the non-greedy qualifier has no effect.  By default, regex's are greedy, 
so if your string looked like this:

'<b><lightblue>name:</></b>xxxxxxxxxxxxxxx</>'

then the greedy version of your regex:

/>(.+)<\/>/   <----(no '?')

would match:

><lightblue>name:</></b>xxxxxxxxxxxxxxx</>

That's because the portion:

<lightblue>name:</></b>xxxxxxxxxxxxxxx

is interpreted as "any character(.) one or more times(+)".

On the other hand, your non-greedy regex(i.e. with the '?') would match:

<lightblue>name:</>


If you examine your string again:

'<b><lightblue>name:</></b>'

the 'lightblue' substring is preceded by the characters '><', and that 
is different from what precedes 'b'.  You can use that fact to get 
'lightblue' instead of 'b'.  This regex will get 'lightblue':

><([^>]+)

That says to look for '><' followed by one or more characters that are 
not a '>'.  That will match:

'><lightblue'

To get 'name:', you can do something similar.  This is the rest of the 
string after 'lightblue':

'>name:</></b>'

Here is a regex to get 'name:':

>([^<]+)

That says to look for a '>', followed by one or more characters that are 
not a '<'.  Here it is altogether:


pattern =  /><([^>]+)>([^<]+)/
str = "<b><lightblue>name:</></b>"

match_obj = pattern.match(str)
puts match_obj[1]
puts match_obj[2]

--output:--

lightblue
name:



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