Ruby Quicktips

String#gsub with a block generates the substitution string dynamically

String#gsub is a common method for finding and replacing all occurrences of a text in a string. It is often used, like this:

"Where is the needle in the haystack?".gsub('needle', 'NEEDLE')
# => "Where is the NEEDLE in the haystack?"

But gsub can also use regular expressions, like this:

"Where is the needle in the haystack?".gsub(/n\w{5,}/, '*')
# => "Where is the * in the haystack?"

gsub can also take a block which can use all of the global match variables, such as $&, $', $1, $2, to build up a replacement string. This allows for replacement strings to be generated using information from the match, like this:

"Where is the needle in the nefarious haystack?".gsub(/n\w{5,}/) do
  '*' + $&.upcase + '*'
end
# => "Where is the *NEEDLE* in the *NEFARIOUS* haystack?"

Blocks provide gsub with the ability to generate the substitution string using information about a match that is only available after a match is identified. You can use any of the global variables associated with matches from within the block.

This tip was submitted by Tim Rand.