Question

What exactly is the line row[/\w+/] doing in the following situation?

fields = rows.shift.map{ |row| 
  row[/\w+/]
}

I understand that the first value is removed from rows, its values are all passed into the block and replaced by the value returned from the block. But what is [/\w+/] doing to the value? In this case each row is an array of alphanumeric strings.

Answer 1

You can see how the string index operator [] works here.

For example, if you pass a number (row[5]) it returns a substring containing the character at position 5 (0 being the first). But when a regular expression is used instead of a number, it returns the first substring that matches the regex.

In this particular case the regex is matching one or more word characters A-Z, a-z, 0-9, and _

"abcd efg hij"[/w+/] #=> "abcd"
"abcd_efg hij"[/w+/] #=> "abcd_efg"