# Regular expressions

**URL:** <https://www.ruby-forum.com/t/regular-expressions/70798>\
**Category:** Ruby\
**Created:** [October 3, 2006, 2:02am UTC](https://www.ruby-forum.com/t/regular-expressions/70798 "2006-10-03T02:02:23Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 2:02am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/1 "2006-10-03T02:02:23Z")

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I’m sorry if this has been done before, but I can’t seem to find any  
good examples:

I’m trying to search mail messages for headers based on the following:

config = YAML.load\_file(‘config.yaml’)

$stdin.each do |line|  
puts line if line =~ /^Recevied:/ … line =~ /by  
#{config[‘hostname’]}/o  
end

and guess what? It doesn’t work.  
If I hard code the hostname it works fine.  
But this and /by config[‘hostname’]/ both fail to match.

So, how do you set a variable in a regular expression?

Also, how would I match multiple lines?

I would much rather match something like:

/^Received:.+?\s{4,}by #{config[‘hostname’]}/sm  
because that will pick up the one Received header I want.  
I can write in Perl 5 regex but I’m not as familiar with Ruby’s methods.

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<div class="post-metadata">

**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 2:30am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/2 "2006-10-03T02:30:24Z")

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Tom A. wrote:

> So, how do you set a variable in a regular expression?

You have it right.

> Also, how would I match multiple lines?

You’re right again with the //m. But you can’t read line-by-line  
(IO#each) and match across multiple lines without building some kind of  
parser. It’s usually better to just read the data into a single string  
for multiline matches.

Input (multiline):  
Received: from mail.papa.smurf (mail.papa.smurf [127.0.0.1])  
by removed\_email\_address@domain.invalid

config = {‘hostname’ =\> ‘removed\_email\_address@domain.invalid’}  
line = $stdin.read  
puts line if line =~ /^Received:.\*by #{config[‘hostname’]}/m

> I would much rather match something like:
> 
> /^Received:.+?\s{4,}by #{config[‘hostname’]}/sm  
> because that will pick up the one Received header I want.  
> I can write in Perl 5 regex but I’m not as familiar with Ruby’s methods.

Input (single line):  
Received: from mail.papa.smurf (mail.papa.smurf [127.0.0.1]) by  
removed\_email\_address@domain.invalid

$stdin.each { |line|  
puts line if line =~ /^Received:.+?\s{4,}by #{config[‘hostname’]}/  
}

Regards,  
Jordan

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<div class="post-metadata">

**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 2:40am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/3 "2006-10-03T02:40:25Z")

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MonkeeSage wrote:

> for multiline matches.
> 
> Input (multiline):  
> Received: from mail.papa.smurf (mail.papa.smurf [127.0.0.1])  
> by removed\_email\_address@domain.invalid
> 
> config = {‘hostname’ =\> ‘removed\_email\_address@domain.invalid’}  
> line = $stdin.read  
> puts line if line =~ /^Received:.\*by #{config[‘hostname’]}/m

This prints out the entire message.

I’m only trying to get the one section that matched the Received header.  
I should be able to do with with the statement  
puts line if line =~ /Received/ … line =~ /#{config[‘hostname’]}/  
At least that’s what I’m believe I’m being led towards.

But the match on /Received/ turns on the printing and it never matches  
on the  
second regexp.

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<div class="post-metadata">

**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 2:40am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/4 "2006-10-03T02:40:25Z")

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Tom A. wrote:

> end
> 
> and guess what? It doesn’t work.

Yes, it doesn’t, and I think I know why. I have been hearing for years  
how  
it just doesn’t matter whether young people learn how to spell common  
words, and I have steadfastly taken the position that it does matter.

It turns out that people who can’t spell, and who leave school confident  
that it is an out-of-date art, slowly discover all the odd places where  
knowing how to spell actually makes a difference.

So, now that you are living in reality, perhaps you will discover one of  
the  
problems with your regular expression is that “Recevied” isn’t a word,  
and  
it won’t filter out those e-mails you want to collect.

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<div class="post-metadata">

**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 3:10am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/5 "2006-10-03T03:10:26Z")

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Tom A. wrote:

> > parser. It’s usually better to just read the data into a single string
> 
> This prints out the entire message.

Well, if the regexp as written correctly identifies the beginning and  
end of  
the desired area, how about this:

result = line.sub(/^(Received:._?by #{config[‘hostname’]})._/m,"\1")

puts result if result.size \> 0

Note the question mark in the middle of the regexp. It means to stop at  
the  
first match, not the last, of what follows. This change may not matter  
in  
practice, but it is a good habit to fall into when dealing with a lot of  
data. One special case might fail after thousands of successful matches.

This hasn’t been tested.

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<div class="post-metadata">

**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 4:24am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/6 "2006-10-03T04:24:31Z")

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On 2006.10.03 10:20, MonkeeSage wrote:

> puts $1 if line =~ /^Received:(.+?)\s{4,}by #{config[‘hostname’]}/
> 
> # prints " from mail.papa.smurf (mail.papa.smurf [127.0.0.1])"
> 
> }

And you almost certainly want to Regexp.escape that interpolation.

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**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 3:20am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/7 "2006-10-03T03:20:26Z")

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Tom A. wrote:

> I’m only trying to get the one section that matched the Received header.  
> I should be able to do with with the statement  
> puts line if line =~ /Received/ … line =~ /#{config[‘hostname’]}/  
> At least that’s what I’m believe I’m being led towards.

In ruby … is a range operator. If you want a grouped match, use parens  
and a backreference just like perl.

$stdin.each { |line|  
puts $1 if line =~ /^Received:(.+?)\s{4,}by #{config[‘hostname’]}/

# prints " from mail.papa.smurf (mail.papa.smurf [127.0.0.1])"

}

Regards,  
Jordan

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<div class="post-metadata">

**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 3:39pm UTC](https://www.ruby-forum.com/t/regular-expressions/70798/8 "2006-10-03T15:39:43Z")

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On 10/3/06, Tom A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> and guess what? It doesn’t work.  
> If I hard code the hostname it works fine.  
> But this and /by config[‘hostname’]/ both fail to match.

To eliminate a possibility, do

p config[‘hostname’]

just before the $stdin line there. I can’t tell you how many times I  
have forgotten about invisible newlines tacked onto my variables that  
cause all sorts of matching to fail.

Les

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**Author:** ![tma](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ed655f/32.png) [@tma](https://www.ruby-forum.com/u/tma)\
**Post date:** [October 3, 2006, 11:40am UTC](https://www.ruby-forum.com/t/regular-expressions/70798/9 "2006-10-03T11:40:54Z")

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Paul L. wrote:

> > puts line if line =~ /^Recevied:/ … line =~ /by  
> > #{config[‘hostname’]}/o  
> > end
> > 
> > and guess what? It doesn’t work.
> 
> Yes, it doesn’t, and I think I know why. I have been hearing for years how  
> it just doesn’t matter whether young people learn how to spell common  
> words, and I have steadfastly taken the position that it does matter.

You make a pretty good example of another lost art…