CODEWITHSUNDEEP
pythonregexpython-3.x
LandSharks
LandSharks

Reputation: 437

What exactly do the flags do in the re library for python 3?

I am unable to find explanations to what exactly the flags due for certain operations within the re library.

https://docs.python.org/3.6/library/re.html

for example, I have the following bit of python code:

nestedPar = re.findall(r"\([^\(\)]*\)", s, 0)

but I am unsure what exactly is happening when I change that 0 to a 1, since it results in a crash.

Can anyone explain what this flag means exactly?

Upvotes: 1

Views: 495

Answers (1)

zwer
zwer

Reputation: 25789

The third argument is options bit mask, and bit masks are defined as (this, technically, may vary from version to version):

SRE_FLAG_TEMPLATE = 1 # template mode (disable backtracking)
SRE_FLAG_IGNORECASE = 2 # case insensitive
SRE_FLAG_LOCALE = 4 # honour system locale
SRE_FLAG_MULTILINE = 8 # treat target as multiline string
SRE_FLAG_DOTALL = 16 # treat target as a single string
SRE_FLAG_UNICODE = 32 # use unicode locale
SRE_FLAG_VERBOSE = 64 # ignore whitespace and comments
SRE_FLAG_DEBUG = 128 # debugging

Using a simple bitwise OR (or your regular +) you can combine multiple flags (e.g. re.MULTILINE | re.DOTALL). It shouldn't fail on 1 but please don't use numbers directly, they are assigned to meaningful constants for a reason.

EDIT - Template mode (re.T or re.TEMPLATE), whose flag is 1 as shown above, is experimental and a lot of things can go wrong with it, directly from the source:

# sre extensions (experimental, don't rely on these)
T = TEMPLATE = sre_compile.SRE_FLAG_TEMPLATE # disable backtracking

So you've encountered a problem with the templating mode. Either way for happy life, do not set the flags yourself :)

Upvotes: 3

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