Named Captures in Ruby Regular Expressions

In Ruby we can use the match method which belongs to the String class to both check a string for a regular expression match but also to extract data using captures.

Captures allow you to extract part of the string... to isolate it. You do this by including () parentheses, with whatever that is inside the parentheses being captured.

Normal Captures

We can see an example below of a normal capture which doesn't assign a name or key to the value being extracted.

BALANCE_REGEX = %r{\A$(\d{1,}\.\d{2})\Z}
value = "$123.55"
if matches = value.match(BALANCE_REGEX)
  matches[1]
else
  '0.00'
end

This is fine, but it isn't as clear as it could be. What are we extracting? Is it index 0, 1, or something else? Here is where named captures arrive on the scene.

Named Captures

What if we could assign a name/key to the value being captured/extracted from our string? Well in Ruby it is easy. Inside of our parentheses we can begin with this: ?<balance>. It will store whatever is captured by the key between the <>.

BALANCE_REGEX = %r{\A$(?<balance>\d{1,}\.\d{2})\Z}
value = "$123.55"
if matches = value.match(BALANCE_REGEX)
  matches[:balance]
else
  '0.00'
end

If we take a look at the matches variable we'll see that it doesn't just put our capture into #<MatchData "$123.55" balance:"123.55">.

We can then easily access it by its key matches[:balance]. Much better!