re = /^[\d\/ :]+:[^:]+:(.*)$|(.*)$/m
str = '11/04/2015 11:45:01: James: Cheers guys, enjoy the weekend!
11/04/2015 12:08:55: Sarah: Sounds good James
11/04/2015 12:09:24: Sarah: What are the details of the trip?
11/04/2015 12:19:06: Leah: Driving up on Friday.
Saturday we\'ll hit the beach.
Sunday paaaaarty!
11/04/2015 12:29:54: James: Nice.'
subst = '$1$2'
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