re = /(.*(?:ST|AVENUE|STREET)\b).*/m
str = '9999 ST PAUL ST BSMT
GARRISON BL & BOARMAN AVENUE REAR
1234 MAIN STREET 123
1234 MAIN ST UNIT1'
subst = '\\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