[racket] Question about string conversion

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Thu Jul 18 07:04:28 EDT 2013

Also, check out the net/head library:



On Thu, Jul 18, 2013 at 2:25 AM, Carl Eastlund <cce at ccs.neu.edu> wrote:

> Chris,
> The regexp-match* function has a rather complex input/output
> specification, because it is doing a lot of things at once.  If you only
> want the first match of a regular expression, you might want to consider
> regexp-match instead (no *); its specification is slightly simpler, though
> still a little complex.
> The car function, on the other hand, is very straightforward.  Given a
> list, it extracts and returns the first element of that list.  The name
> "car" is rather anachronistic; Racket also has a function called first that
> does the same thing.  If it makes things clearer to you, use that one
> instead.
> If using functions like regexp-match* and car is giving you trouble in
> your program, I suggest playing around with them in racket or DrRacket,
> outside of any program.  Just apply regexp-match*, car, and any other
> functions you want to learn more about, to different inputs and see what
> results you get.  That should give you a clearer idea of how to put them
> together in your program.  It will also give you a starting point for
> asking more concrete questions about specific inputs and outputs the rest
> of us can look at, try ourselves, and explain.
> For instance, see what (regexp-match* "[abc]+" "banana") produces.  Also
> see what (car (list 1 2 3)) produces.  Try string->list and list->string if
> you're curious to see how they work, too.
> When you've got a handle on how each function works individually, you can
> get back to your program.  Write lots of tests for your program, and break
> your program up into smaller "helper" functions, and write tests for them
> too.  All these steps will make it much easier to see what each function is
> doing, and if the output isn't what you expected it to be, these steps will
> help you figure out where things went wrong and why.
> Thanks for your question, feel free to ask more if you have them.  Good
> luck!
> Carl Eastlund
> P.S. I know this isn't a simple "regexp-match* works like this" answer.
> Your question is actually several questions -- what kind of output does
> regexp-match* produce, how does one convert between lists and strings, how
> does the function car work, and so forth.  I hope the process above will
> help you work through some of the complexity and get to some simpler, more
> direct questions that are easier to tackle.  Sorry if this feels like a bit
> of a runaround, but I think you'll get a handle on it pretty quick!
> On Thu, Jul 18, 2013 at 2:29 AM, <m0nastic at tengulabs.com> wrote:
>> Hi, I'm super new to Racket, but had what I hope isn't too
>> embarrassingly easy of a question.
>> I'm writing a program that parses SMTP headers in order to automate some
>> business-y workflow. I managed to put together a series of
>> "regexp-match*"s to parse out the specific identifiers I was looking for
>> (although truth be told, mail headers are surprisingly nonstandard even
>> within a single message), and ran into an error later when trying to
>> string-append them. I eventually discovered by process of elimination
>> (after trying string?, etc. to figure out how the results of a regexp
>> are stored) that they were a list. I tried list->string and a few other
>> methods to covert them, but wasn't having any luck.
>> I eventually discovered that I could sort of cheat, by just wrapping the
>> regexp-match function with a car (which worked, because this particular
>> list only had one element), and then it was usable from then on (and
>> validated true from "string?").
>> So, my actual question is two-fold. First, It seems like I must have
>> missed some pretty obvious way to capture the initial regex match as a
>> string, but I'm not sure what it was.
>> And secondly, and actually what I'm more curious about, why does it
>> become a string when wrapped by car? I would have presumed that car
>> would leave structures as a list.
>> Again, I apologize if this is something dead-simple, but I really did
>> spend a good portion of the day stuck in "regex-hell", and so was
>> curious.
>> -Chris
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