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RCR 150: String#start_with? / #end_with?

submitted by anonymous on 2003-08-21 10:56:56

This is a legacy RCR. If this is your RCR, please resubmit it using the new format and process.

I'd like the following to be in Standard Ruby:

class String
  def start_with?(other, length = other.length)
    self[0...length] == other[0...length]
  end

  def end_with?(other, length = other.length)
    self[-length..-1] == other[-length..-1]
  end
end

This allows the more readable "foobar".start_with? "foo" and "barqux".end_with? "qux" instead of the more long winded and error prone "foobar"[0...3] == "foo" and "barqux"[-3..-1] == "qux" .

IMHO checking whether a String starts or ends with another String is done commonly so there should be a simple and clear way to do so.

----
=== Add comments here
<h4 class="commentheader">Re: String#start_with? / #end_with? (, 2003-08-21 22:26:43)</h4>
<div class="commentbody">How about:<br />
str = "foobar"<br />
<br />
if (str =~ /^foo/)<br />
...<br />
end<br />
<br />
if (str =~ /bar$/)<br />
...<br />
end<br />
</div>
<h4 class="commentheader">It's not as easy as that... (, 2003-08-22 00:43:44)</h4>
<div class="commentbody">The real solution which allows you to use arbitrary strings would have to be more like <code>str =~ /#{Regexp::quote("a string...")}$/</code> and I don't think that's very idiomatic.</div>
<h4 class="commentheader">Re: It's not as easy as that... (, 2003-08-22 11:13:22)</h4>
<div class="commentbody">You don't have to use Regexp::quote(...):<br />
<br />
start_with = "foo"<br />
end_with = "baz"<br />
end_with2 = "bar baz"<br />
<br />
str = "foo bar baz"<br />
<br />
puts "#{str} starts with '#{start_with}'" if str =~ /^#{start_with}/<br />
puts "#{str} starts with 'foo bar'" if str =~ /^foo bar/<br />
puts "#{str} ends with '#{end_with}'" if str =~ /#{end_with}$/<br />
puts "#{str} ends with '#{end_with2}'" if str =~ /#{end_with2}$/<br />
puts "#{str} ends with 'bar baz'" if str =~ /bar baz$/<br />
<br />
This will produce:<br />
foo bar baz starts with 'foo'<br />
foo bar baz starts with 'foo bar'<br />
foo bar baz ends with 'baz'<br />
foo bar baz ends with 'bar baz'<br />
foo bar baz ends with 'bar baz'<br />
<br />
You can even check start_with and end_with in one shot by something like:<br />
<br />
str =~ /^foo.*?baz$/<br />
<br />
<br />
<br />
</div>
<h4 class="commentheader">Re: It's not as easy as that... (, 2003-08-22 19:02:00)</h4>
<div class="commentbody"><pre>start_with = "*** make money fast ***"
str = "Hello World!"
puts "#{str} starts with '#{start_with}'" if str =~ /^#{start_with}/</pre>

<p>produces this error for me:</p>

<p>RegexpError: invalid regular expression; there's no previous pattern, to which '*' would define cardinality at 2: /^*** make money fast ***/</p>

So the Regexp::quote() <strong>is</strong> needed.</div>
<h4 class="commentheader">Re: It's not as easy as that... (, 2003-08-22 22:50:46)</h4>
<div class="commentbody">Bummer...how did I miss that? I guess I never had a single case where the string contained a special character...but hey, it's not a bad thing that I learned something from somebody.<br />
<br />
I wish I could change my previous vote to a YES, but (surprise!) the system won't allow me to re-vote/change...sorry about that... :-(</div>
<h4 class="commentheader">Re: It's not as easy as that... (, 2003-08-24 00:59:14)</h4>
<div class="commentbody">Heh, that's no problem. We have 4 out of 5 positives and I guess matz actually reads the comments. :)</div>
<h4 class="commentheader">Re: String#start_with? / #end_with? (, 2003-09-02 19:50:23)</h4>
<div class="commentbody">perhaps starts_with is a better name that start_with<br />
<br />
see <br />
<br />
http://java.sun.com/j2se/1.3/docs/api/java/lang/String.html#startsWith(java.lang.String)</div>
<h4 class="commentheader">Re: String#start_with? / #end_with? (, 2003-09-05 10:00:08)</h4>
<div class="commentbody">That might be the case. -- I was just choosing start_with? because I wanted to stay consistent with some original methods. (mainly Array#include?)</div>

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