anomalous_regex

before = "2015-08-13"
after = "2015-08-25"
full_path = "test.json"
with open(full_path) as infile:
    data = infile.read()
    data = re.sub(r"(var beforeDate = new Date\().*?(\))", "\\1"+before+"\\2", data)
    data = re.sub(r"(var afterDate = new Date\().*?(\))", "\\1"+after+"\\2", data)
with open(full_path, "w") as outfile:
    outfile.write(data)

and the contents of test.json are the following

\n    var beforeDate = new Date('2015-08-21');\n    var afterDate = new Date('2015-08-19');\n

and the output i get is the following

\n    P15-08-13);\n    P15-08-25);\n