newbie question: parse a variable inside an RE?

[John Machin]

On Dec 2, 8:56 am, "Diez B. Roggisch" <de... at nospam.web.de> wrote:
> joemacbusin... at gmail.com schrieb:
>
> > Hi All,
>
> > How do I parse a variable inside an RE?
> > What is the re.search() syntax when your
> > search string is a variable?
> > It's easy to parse hardcoded RE's but not
> > if you use a variable.
>
> Both are exactly equal in difficulty.
>
>
>
>
>
> > Here is my code, input and runtime:
>
> > $ cat test45.py
> > #!/usr/bin/python
>
> > import re
>
> > resp = raw_input('Selection: ')
> > newresp = resp.strip()
> > print "you chose ", newresp
>
> > fname = open('test44.in')
> > for I in fname:
> > #    if re.search('^newresp', "%s"%(I)):     # returns nothing
> > #    if re.search(^newresp, "%s"%(I)):       # syntax error
> >     if re.search("^newresp", "%s"%(I)):      # returns nothing
> >         print I,
>
> How should python know that you want the newresp being expanded? And not
> that you want to search for the word "newresp"?
>
> You need to use the *variable* newresp:
>
> if re.search(newresp, I): ...
>
> If you want to alter it to have a "^" prepended before you use it, you
> need to do so:
>
> newresp = "^" + newresp
>
> And as show above,
>
> "%s" % I
>
> is nothing but I - no need for the string-interpolation.

In fact what the OP is trying to do amounts to a convoluted version of
I.startswith(newresp) which probably isn't his real requirement
anyway :-(