The Coding Way - String#extract - Simpler regex matching

Monday, August 10, 2009

String#extract - Simpler regex matching

String#extract is an extension to the core String class that simplifies extracting values from a string using regular expressions.

The code

First, let me show you the method:

# Returns the various captured elements from this string with the regex applied.
#   Usage:  a,b = 'abc123 is cool'.extract(/([a-z]*)([0-9]*)/)
#   Result: a = 'abc', b = '123'
# With no capture in regex, returns full match
# If no match, returns nil
def extract(regex)
  data = self.match(regex)
  return nil unless data
  if data.size > 1
    return *(data.to_a[1..-1])
  else
    return data[0]
  end
end

So, what does it do?

I always find using String#match and its Regex equivalents to be tedious and hard to follow.

Here's an example of trying to pull the various phone fields out of a user-entered phone number:

# A string, and a regex to parse it out
number = "(800) 555-1212"
regex = /([0-9]{3})?[^0-9]*([0-9]{3})[^0-9]*[0-9]{4}/
# This next bit is a bit verbose for my taste
match = number.match(regex)
if match
  area = match[1]
  prefix = match[2]
  suffix = match[3]
end

Instead, check out using the extract method:

# Same setup
number = "(800) 555-1212"
regex = /([0-9]{3})?[^0-9]*([0-9]{3})[^0-9]*[0-9]{4}/
# Short, sweet, and easy to read
area, prefix, suffix = number.extract(regex)

Basically, #extract allows you to use a regex to pull a value out of a string instance, cleanly. Here are a few more examples:

"Hey, Rob!  Cool method!".extract(/R[a-z]*/)
# "Rob"
"$2,700.00".extract(/([0-9,]+)\.([0-9]{2})/)
# ["2,700", "00"]
first, last = "Rob Morris".extract(/([a-z]+)\s+([a-z]+)/i)
# first = "Rob", last = "Morris"

So there you have it. Extract a single value with a plain regular expression, or one or more values using regexen with capture groups in them. And the syntax is clean and elegant. Hope you like it!