re = /^\S+\h\S+\K\h/m
str = '20 DE NOVIEMBRE
CENTRO
EL ARENAL 4A SECCION
IGNACIO ZARAGOZA
JARDIN BALBUENA
MOCTEZUMA 2A SECCION
MORELOS
PEON DE LOS BAOS'
subst = '\\n'
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