Parse timestamp string

[Reiner Merz]

Hi,

I'm looking for advice on how to parse a timestamp string
according to the ISO 8601 specification.

For those unfamiliar with the standard, here's an example:

2003-09-09T23:00:00Z

A compact form of it can as well be without the minuses and
the colons or with timezone information.

The parser should be small and fast and fairly easy to
integrate into C. And if possible, I don't want to link to an
additional library.

I thought so far about doing it in lex/yacc (flex/bison) but
after thinking it through, I don't consider them to be the right
tools for the job.

Regexes would be exactly what I need but I don't want to have
a whole regex engine that calculates the pattern at runtime when
it's not necessary. There must be a cheaper way of doing it.

I've already searched the internet for a kind of regex
compiler/generator that would generate C code for my specific
regex but unfortunately I didn't find anything similar to what I
want.

Of course, I can do it the hard way by messing around with
the string functions in string.h but if I can avoid that I'll do
so.

Any suggestions are appreciated.

Thanks

Reiner

[Kevin Easton]

Reiner Merz <lo******@gmx.d e> wrote:

Hi Irrwahn,

You don't want to:
- use lex/yacc (which, IMHO, are well suited to produce a
scanner/parser; that's what they're made for)

You must have got me wrong there.

I did try to use lex/yacc because I thought they were made
for these kind of problems, but I didn't succeed. But my
inability to do it could be due to the fact that I've only
rarely used lex/yacc and when I used it, I used it for simple
configuration files. So, you could say, I'm not really
experienced.

My problem was mainly the scanner. I wanted to define my tokens

YEAR as four digits
MONTH as two digits
DAY as two digits
HOUR as two digits
MINUTE as two digits
SECOND as two digits

So, how can the scanner possibly distinguish between MONTH,
DAY, HOUR, MINUTE and SECOND? The flex manual says it picks the
longest possible match and if there are equally long matches it
picks the first defined token.

Ok, I could define a FOURDIGIT and a TWODIGIT token and then
assign the right values in bison. I guess that would work for

2003-09-09T23:00:00Z

but for

20030909T230000 Z (which is a legal date time according to the
ISO specification)

I would imagine, lex gives me

FOURDIGIT 2003
FOURDIGIT 0909
FOURDIGIT 2300
TWODIGIT 00

and that is not really what I want.

comp.compilers is probably a much better place to ask lex/yacc
questions. However, I can tell you that the solution is to just
define a single DIGIT token, then use yacc to recognise four DIGIT
tokens followed by an optional DASH token etc etc...

- Kevin.