[Tutor] Regular Expression Misunderstanding

[Kent Johnson]

Steve Nelson wrote:
> Incidentally continuing my reading of the HOWTO I have sat and puzzled
> for about 30 mins on the difference the MULTILINE flag makes.  I can't
> quite see the difference.  I *think* it is as follows:
>
> Under normal circumstances, ^ matches the start of a line, only.  On a
> line by line basis.
>
> With the re.M flag, we get a match after *any* newline?
>
> Similarly with $ - under normal circumstances, $ matches the end of
> the string, or that which precedes a newline.
>
> With the MULTILINE flag, $ matches before *any* newline?
>
> Is this correct?
I'm not sure, I think you are a little confused. MULTILINE only matters 
if the string you are matching contains newlines. Without MULTILINE, ^ 
will match only at the beginning of the string. With it, ^ will match 
after any newline. For example,
In [1]: import re

A string  containing two lines:
In [2]: s='one\ntwo'

The first line matches without MULTILINE:
In [3]: re.search('^one', s)
Out[3]: <_sre.SRE_Match object at 0x00C3E640>

The second one does not (result of the search is None so nothing prints):
In [4]: re.search('^two', s)

With MULTILINE ^two will match:
In [5]: re.search('^two', s, re.MULTILINE)
Out[5]: <_sre.SRE_Match object at 0x00E901E0>

Kent