Manipulating strings in C

Question

I have a text file in which contains many emails, at the beginning of each email is 3 lines of header information, these include From:, Subject:, Date:. I know that after every ctrl-L character are the header lines, hence the c==12 line.

Currently my from array gets 1 line of text either something like:

From: Rollen Awen <[email protected]>

or

From: [email protected]

So right now I am trying to use delimiters to only keep the email address, but im not sure how to go about it. I have to be able to handle any type of situation, whether its enclosed within < > or if its enclosed between 2 white spaces.

For example, I want to change the

From: Rollen Awen <[email protected]> string into only [email protected]

Or changing

From: [email protected] into [email protected]

...
FILE *emaildata = fopen (argv[1], "r");

    while((c=fgetc(emaildata))!=EOF){
            if(c==12){
                numberemails++;
                fgets(nothing, sizeof(nothing), emaildata);
                fgets(from, sizeof(from), emaildata);
                fgets(subject, sizeof(subject), emaildata);
                fgets(date, sizeof(date), emaildata);
                //printf("%s", from);
            }
    ...

One solution can be: search the line for '@' symbol. once you find it come back till you get 'valid email address chars' and then again start from there till you are getting 'valid email address chars'! But I doubt if this is an efficient solution! — Nullpointer
– Nullpointer, Commented Oct 16, 2014 at 2:46
I honestly think it's a good one. I am not sure how complicated a proper email address can be, but if it's limited only to some character sets it is easy and efficient. I would also consider a regexp library if more logic is needed. — luk32
– luk32, Commented Oct 16, 2014 at 2:55
It might also be important if the input is always valid. E.g. There is at most one email address per line and it's a valid email address. — luk32
– luk32, Commented Oct 16, 2014 at 3:10

John Zwinck · Accepted Answer · 2014-10-16 03:35:02Z

3

This requires memrchr() which glibc gives you if you #define _GNU_SOURCE. If you don't have that function I'm sure you can find a similar one or write it yourself.

// input is either like "John Smith <[email protected]>" or "[email protected]"
// leading and trailing whitespace is skipped
// email is an out-param, must be an array at least as long as input
void parse_email_address(const char* input, char* email)
{
  // skip leading whitespace
  while (isspace(*input)) {
    ++input;
  }

  size_t len = strlen(input);

  // ignore trailing whitespace
  while (len > 0 && isspace(input[len - 1])) {
    --len;
  }

  // parse friendly addresses like "John Smith <[email protected]>"
  // '>' must come last, and '<' must come before it
  if (len > 0 && input[len - 1] == '>') {
    const char* left = memrchr(input, '<', len);
    if (left) {
      len -= left - input + 2; // 2 for '<' and '>'
      input = left + 1;
    }
  }

  memcpy(email, input, len);
  email[len] = '\0';
}

edited Oct 16, 2014 at 3:35

answered Oct 16, 2014 at 3:13

John Zwinck

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5 Comments

luk32 Over a year ago

Ummm, but it doesn't handle address-only lines like From: [email protected], does it? I might have missed something. If I'm right, I think "find @ and span from there" is more robust.

John Zwinck Over a year ago

@luk32: it assumes you've chopped off From: already. Of course that part is easy, but I should have been clearer about it. I'll add a comment.

luk32 Over a year ago

Oh, I thought input is the line. But won't it trip, when there is no < and >? EDIT: OK, I think I see it. Sorry for getting confused. It's a nice counter part to the "find @ and span from it" idea which is also nice IMO.

John Zwinck Over a year ago

@luk32: My concern about the @ finding method was, what if someone used Bobby T@bles <[email protected]>? I think at a minimum you'd need to find the @ from the rear, and even then it seems a bit fraught.

luk32 Over a year ago

Ha, I only exploited email addresses, well thought sir. +1.

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