This python script will look for a regexp pattern in whatever is piped to it, and echo back a Team City service message to set this as a build time variable using a service message.

tc-grep-set-parameter.py

import fileinput
import sys
import re

valuePair = sys.argv[1]
name = valuePair.split("=")[0]
valuePattern = valuePair.split("=")[1]

if not (name and valuePattern):
	print("Call like this 'echo \"Lorem ipsum doler sit\" | python tc-grep-set-parameter.py 'jibberish=(ipsum)'")
	exit(1)

print("Will look for pattern %s for value: %s"% (valuePattern, name))
regexp = re.compile(valuePattern)

input_lines = fileinput.input(sys.argv[2:])
for output_line in input_lines:
	sys.stdout.write(output_line)
	match = regexp.search(output_line)
	if match:
		print("##teamcity[setParameter name='%s' value='%s']"% (name, match.groups()[0]))