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copied!<p>Cribbing from the documentation (<a href="http://ruby-doc.org/core/classes/String.html#M001185" rel="nofollow">http://ruby-doc.org/core/classes/String.html#M001185</a>), here are answers to your two questions "why is the return value 'one t-three'" and "what does { $1[0, 1] } mean?"</p> <p><strong>What does { $1[0, 1] } mean?</strong> The method String#sub can take either two arguments, or one argument and a block. The latter is the form being used here and it's just like the method Integer.times, which takes a block:</p> <pre><code>5.times { puts "hello!" } </code></pre> <p>So that explains the enclosing curly braces.</p> <p>$1 is the substring matching the first capture group of the regex, as described <a href="http://blog.purifyapp.com/2010/01/04/cryptic-ruby-global-variables-and-their-meanings/" rel="nofollow">here</a>. [0, 1] is the string method "[]" which returns a substring based on the array values - here, the first character.</p> <p>Put together, { $1[0, 1] } is a block which returns the first character in $1, where $1 is the substring to have been matched by a capture group when a regex was last used to match a string.</p> <p><strong>Why is the return value 'one t-three'?</strong> The method String#sub ('substitute'), unlike its brother String#gsub ('globally substitute'), replaces the first portion of the string matching the regex with its replacement. Hence the method is going to replace the first substring matching "(t\w*)" with the value of the block described above - i.e. with its first character. Since 'two' is the first substring matching (t\w*) (a 't' followed by any number of letters), it is replaced by its first character, 't'.</p>

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