slre/README.md

SLRE: Super Light Regular Expression library

SLRE is an ISO C library that implements a subset of Perl regular expression syntax. Main focus of SLRE is small size, simple API, clarity of code and extensibility. It is making it perfect for tasks like parsing network requests, configuration files, user input, etc, when libraries like PCRE are too heavyweight for the given task. Developers in embedded would benefit most.

Extensibility is another great aspect of SLRE. For example, if one wants to introduce a new metacharacter, '\i', meaning 'IPv4 address', it is easy to do so with SLRE.

Supported Syntax

^        Match beginning of a buffer
$        Match end of a buffer
()       Grouping and substring capturing
\s       Match whitespace
\S       Match non-whitespace
\d       Match decimal digit
+        Match one or more times (greedy)
+?       Match one or more times (non-greedy)
*        Match zero or more times (greedy)
*?       Match zero or more times (non-greedy)
?        Match zero or once
x|y      Match x or y (alternation operator)
\meta    Match one of the meta character: ^$().[]*+?|\

Not supported but in progress:

[...]    Match any character from set
[^...]   Match any character but ones from set
\xDD     Match byte with hex value 0xDD

API

int slre_match(const char *regexp, const char *buf, int buf_len,
               struct slre_cap *caps, const char **error_msg);

slre_match() matches string buffer buf of length buf_len against regular expression regexp, which should conform the syntax outlined above. If regular expression regexp contains brackets, slre_match() will capture the respective substrings. Array of captures, caps, must have at least as many elements as number of bracket pairs in the regexp.

slre_match() returns 0 if there is no match found. Otherwise, it returns the number scanned bytes from the beginning of the string. This way, it is easy to do repetitive matches. Hint: if it is required to know the exact matched substring, enclose regexp in a brackets and specify caps, which should be an array of following structures:

struct slre_cap {
  const char *ptr;  /* Points to the matched fragment */
  int len;          /* Length of the matched fragment */
};

Example: parsing HTTP request

const char *error_msg, *request = " GET /index.html HTTP/1.0\r\n\r\n";
struct slre_cap caps[4];

if (slre_match("^\\s*(\\S+)\\s+(\\S+)\\s+HTTP/(\\d)\\.(\\d)",
               request, strlen(request), caps, &error_msg)) {
  printf("Method: [%.*s], URI: [%.*s]\n",
         caps[0].len, caps[0].ptr,
         caps[1].len, caps[1].ptr);
} else {
  printf("Error parsing [%s]: [%s]\n", request, error_msg);
}