Ruby Regexp vs Perl and C#

[Chris Meyers]

I am having trouble with the Ruby regexp engine and don't know if it is
my lack of experience with Ruby, or if it just isn't possible to do
certain things with the Ruby engine. Basically I have the following
code in perl and C#, simplified for the example:

Perl:
$line = "banana";
while( $line =~ /(an)*/g )
{
print $`. "<<" . $& . ">>" . $' . "\n";
}

C#:
using System;
using System.Text.RegularExpressions;

public class MyClass
{
public static void Main()
{
string testString = "banana";
MatchCollection matches = Regex.Matches( testString, "(an)*" );
foreach( Match match in matches)
{
Console.WriteLine( testString.Substring( 0, match.Index) +
"<<" + match.Value + ">>" + testString.Substring( match.Index +
match.Length));
}
}
}

Output from both:
<<>>banana
b<<anan>>a
banan<<>>a
banana<<>>

While the C# way of getting the answer is a bit more messy, it is still
possible. I am wondering if/how to do this in Ruby. From what I have
seen, all the Ruby options are a single match which doesn't help me,
especially with the given example where Ruby's only match would be the
one before the string thus being empty. Using the string#scan in ruby
also doesn't help as it gives a 2 dimensional array of the following:
Ruby:
irb(main):001:0> line = "banana"
=> "banana"
irb(main):002:0> line.scan(/(an)*/)
=> [[nil], ["an"], [nil], [nil]]

So while Ruby finds that there should be 4 matches, it returns them in a
difficult to use format, with no index information, and matches only
"an" rather than "anan" as I want.

So basically is there a way to do this in Ruby? Which can also be asked
as, how do you do multiple matches in Ruby with full match information?

Thanks,
Chris

 

[matt neuburg]

how do you do multiple matches in Ruby with full match information?

gsub

m.

 

[matt neuburg]

gsub

In fact you could effectively write it exactly as you did it in Perl:

"banana".gsub(/(an)*/) {
puts $` + "<<" + $& + ">>" + $'
}

m.

 

[James Edward Gray II]

I am having trouble with the Ruby regexp engine and don't know if
it is
my lack of experience with Ruby, or if it just isn't possible to do
certain things with the Ruby engine. Basically I have the following
code in perl and C#, simplified for the example:

Perl:
$line = "banana";
while( $line =~ /(an)*/g )
{
print $`. "<<" . $& . ">>" . $' . "\n";
}

scan() can also take a block:

"banana".scan { puts "#{$`}<<#{$&}>>#{$'}" }

James Edward Gray II

 

[Verno Miller]

...
... and [Ruby] matches only "an" rather than "anan" as I want.

So basically is there a way to do this in Ruby? Which can also be asked
as, how do you do multiple matches in Ruby with full match information?

Thanks,
Chris

It's a bit tricky but still doable!

line = "banana"

If your goal is to match just (an)* in "banana" use:

line.scan(/((an)*)/) { |str|
#puts $1.inspect
puts $1 if $1 != ""
}


To match surrounding characters as well use:

line.scan(/([^a]|a(?=[^n]|$)|(an)*)/) { |str|
puts $1.inspect unless $1.empty?
}


Additional characters can be inserted this way:

line = "bananaxyzantuia"

str = ""

line.scan(/([^a]|a(?=[^n]|$)|(an)*)/) {

match = $1

puts match if match =~ /^[^a]$/
puts match if match =~ /^a$/
puts match if match =~ /^(an)+$/

if match =~ /^[^a]$/ then str << match
elsif match =~ /^a$/ then str << match
elsif match =~ /^(an)+$/ then str << "<<" << match << ">>"
end

}

puts str #=> b<<anan>>axyz<<an>>tuia


Instead of if-elsif-end you can also use case-when-end of course (cf.
http://www.bigbold.com/snippets/posts/show/1313 ).


Cheers,
verno

 

[Chris Meyers]

It's a bit tricky but still doable!

line = "banana"

If your goal is to match just (an)* in "banana" use:

line.scan(/((an)*)/) { |str|
#puts $1.inspect
puts $1 if $1 != ""
}

Thank you Verno, that is what I needed. By using the block syntax I am
able to get the same output as Perl or C# and I can adapt that to my
problem at hand. For anyone interested, to recreate the same output in
Ruby as my Perl and C# example the following code works:

line="banana"
line.scan(/(an)*/) {
puts $` + "<<" + $& + ">>" + $' + "\n"
}

Thanks again, Chris

 

[Martin DeMello]

line="banana"
line.scan(/(an)*/) {
puts $` + "<<" + $& + ">>" + $' + "\n"
}

Also, if you do a

require 'English'

you can write that as

puts $PREMATCH + "<<" + $MATCH + ">>" + $POSTMATCH

(you don't need the explicit \n if you're using puts)

martin