[Tutor] sneaky re.compile --is this a bug??

[Walter Prins]

Hi Albert,

On 16 August 2012 09:45, Albert-Jan Roskam <fomcl at yahoo.com> wrote:
>
> Hi,
>
> Is it intended behavior that regular expression flags are ignored when
> compiled regexes are used? In the code below, I intend to match path names
> case-independently, but once the flags (in this case: no flags at all) have
> been set in re.compile, subsequent flags in re.search are ignored (no pun
> intended). This is sneaky! Is this because of the old Python version I am
> using?

I don't think so .  Firstly I think you're arguably using the compiled
regex incorrectly.  The normal idiom for a compiled regex is:
compiled_regex = re.compile(....)

and then:
compiled_regex.match(...)
compiled_regex.search(...)

Note, both the regular expression and flags to be used is "baked" into
the compiled regex object.

By contrast, you're calling re.search() and then passing a previously
compiled regex (instead of a regex pattern as strictly speaking is
required) to the re.search() method.  I suspect that what's happening
is that re.search() is perhaps trying to be being helpful by seeing
that it's not been given a regular expression but intead a compiled
regex, and is then therefore relaying the re.search() request back to
the compiled regex's search() method.  But, as mentioned, a compiled
regex includes the flags it was compiled with, which may differ from
the ones passed to re.search(), which is I think why you're seeing
what you're seeing.  (Observe also the compile regex object's search()
method does not accept a flags parameter.)  If you want to use
different flags you must compile another copy of the regex expression
if you want to use the regex in a compiled form.

All of that said, you can actually inspect the flags applicable to a
compiled regex by evaluating the "flags" attribute on the compiled
regex object.  This will report a combination of the flags given to
compile() and any flags specified inline inside the regular expression
itself.  It may be worth enquiring on the Python development list
whether the behaviour around your case of re.search( compiled_regex,
flags) should be perhaps handled slightly differently if the flags
specified do not match the already existing flags in the compiled
regex, perhaps by raising an exception.  The principle of least
surprise would seem to suggest that this might be better than silently
giving you something else than asked for.

Walter