Why does

text = <<ENDTEXT
this is an <field>wibble</field>
of stuff <field>bobble</field>
ENDTEXT

wibble = 2.0
bobble = 3.0

newtext = text.gsub( /<field>(.*?)<\/field>/, "#{\1}" )

puts newtext

throw this error:
./test.rb:13: syntax error, unexpected $undefined
newtext = text.gsub( /<field>(.*?)<\/field>/, "#{\1}" )

and not interpolate to produce
this is an 2.0
of stuff 3.0

?

And how can I do that interpolation?  (adding \s to \1, e.g. \\1
doesn't help)

Many TIA,
Craig