# text i wanna match:

keyone=valueone abc requestClientApplication=Mozilla/5.0 (compatible; heritrix/3.3.0-SNAPSHOT-20150302-2206 +http://127.0.0.1) keytwo=value two abc

# regex used:

requestClientApplication=((?:.(?![a-zA-Z\d]+=))*)

# works on https://regex101.com/#python
# Getting exact full match ("Mozilla/5.0 (compatible; heritrix/3.3.0-SNAPSHOT-20150302-2206 +http://127.0.0.1)" in this case)
# but in python:

for file in os.listdir(os.getcwd()):
    if file.endswith(".log"):
        with open(file) as f:
            for line in f: # line is the example text above
                matchclapp = re.search('requestClientApplication=((?:.(?![a-zA-Z\d]+=))*)', line)
                print(matchclapp)

# Getting this output:

# <_sre.SRE_Match object; span=(136, 273), match='requestClientApplication=Mozilla/5.0 (compatible;>
# Match was cut in the middle...