re = /((\b\d+)(?:\sPI?N ?#? ?))|((?:PI?N ?#? ?)(\d+))/mi
str = '9999XX00999X PIN # 999999
9999XX000999 - PIN#909999
XXXXX XX PIN #9099999
9999XX99X PIN999099
99XX999X PN 9999999
9999999 PIN
90PA999 PN 999999
999009 PIN#
99PA99 PN#9999999
PIN 999990
XXXPIN990999
PIN#999099
PIN# 9990909
PIN#9099999*'
subst = 'test\\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