ruby - What does this regex `str.gsub(/#{(.*?)}/)` do?

.* means any character, so why is the .*? needed in the following?

str.gsub(/\#{(.*?)}/) {eval($1)}

Answer

.* is a greedy match, whereas .*? is a non-greedy match. See this link for a quick tutorial on them. Greedy matches will match as much as they can, while non-greedy matches will match as little as they can.

In this example, the greedy variant grabs everything between the first { and the last } (the last closing brace):

'start #{this is a match}{and so is this} end'.match(/\#{(.*)}/)[1]
# => "this is a match}{and so is this"

while the non-greedy variant reads as little as it needs to make the match, so it only reads between the first { and the first successive }.

'start #{this is a match}{and so is this} end'.match(/\#{(.*?)}/)[1]
# => "this is a match"