Python 2.7.3 (default, Mar 13 2014, 11:03:55) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> str = "2015-05-19T10:34:23.710+0300: 4.317: [GC2015-05-19T10:34:23.710+0300: 4.318: [ParNew: 419456K->13558K(471872K), 0.0193310 secs] 419456K->13558K(5190464K), 0.0195370 secs] [Times: user=0.16 sys=0.01]"
>>> m = re.match(r'GC(\d+)', str)
>>> m.group()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'

import re
data = "2015-05-19T10:34:23.710+0300: 4.317: [GC2015-05-19T10:34:23.710+0300: 4.318: [ParNew: 419456K->13558K(471872K), 0.0193310 secs] 419456K->13558K(5190464K), 0.0195370 secs] [Times: user=0.16 sys=0.01]"
m = re.search(r'GC(\d+)', data)
print(m.group())

m.group(1)