re = /.*(?: DE: (.*))(?: ID: (.*)){0,1}(?: MOTIF: (.*))(?: REF: (.*)){0,1}/m
str = 'PRELEVEMENT EUROPEEN 2001427553 DE: MUTUELLE DE L\'INDUSTRIE DU PETROLE ID: FR69MIP108863 MOTIF: Prelevement 42CXK 024 de MIP du 01/
VIR RECU 0485970259S DE: CPAM VAR MOTIF: 180930001235180930001235 REF: 18093000123'
subst = 'Tiers:\\t$1 \\rid:\\t$2 \\rmotif:\\t$3\\r\\r'
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