What does the o modifier for a regexp mean?

Question

Ruby regexp has some options (e.g. i, x, m, o). i means ignore case, for instance.

What does the o option mean? In ri Regexp, it says o means to perform #{} interpolation only once. But when I do this:

a = 'one'  
b = /#{a}/  
a = 'two'  

b does not change (it stays /one/). What am I missing?

Answer 1

Straight from the go-to source for regular expressions:

/o causes any #{...} substitutions in a particular regex literal to be performed just once, the first time it is evaluated. Otherwise, the substitutions will be performed every time the literal generates a Regexp object.

I could also turn up this usage example:

# avoid interpolating patterns like this if the pattern
# isn't going to change:
pattern = ARGV.shift
ARGF.each do |line|
    print line if line =~ /#{pattern}/
end

# the above creates a new regex each iteration. Instead,
# use the /o modifier so the regex is compiled only once

pattern = ARGV.shift
ARGF.each do |line|
    print line if line =~ /#{pattern}/o
end

So I guess this is rather a thing for the compiler, for a single line that is executed multiple times.