re = /model: ([^\n]+)(.*)bitmap [^\n]+/m
str = '# model: pmy0_footr015
filedependancy Unknown
newmodel pmy0_footr015
setsupermodel pmy0_footr015 NULL
classification Character
setanimationscale 1.0
beginmodelgeom pmy0_footr015
node dummy pmy0_footr015
parent NULL
endnode
node trimesh pmy0_footr015g
parent pmy0_footr015
position 0.0 0.0 0.0
orientation 1.0 0.0 0.0 0.0
bitmap pmh0_footr050
verts 15
-0.0378169 -0.00381612 0.00857995'
subst = 'model: $1$2bitmap $1'
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html