SLRE: Super Light Regular Expression library

SLRE is an ISO C library that implements a subset of Perl regular expression syntax. Main features of SLRE are:

• Written in strict ANSI C'89
• Small size (compiled x86 code is about 5kB)
• Uses little stack and does no dynamic memory allocation
• Provides simple intuitive API
• Implements most useful subset of Perl regex syntax (see below)
• Easily extensible. E.g. if one wants to introduce a new metacharacter \i, meaning "IPv4 address", it is easy to do so with SLRE.

SLRE is perfect for tasks like parsing network requests, configuration files, user input, etc, when libraries like PCRE are too heavyweight for the given task. Developers of embedded systems would benefit most.

Supported Syntax

(?i)    Must be at the beginning of the regex. Makes match case-insensitive
^       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 (non-greedy)
x|y     Match x or y (alternation operator)
\meta   Match one of the meta character: ^$().[]*+?|\
\xHH    Match byte with hex value 0xHH, e.g. \x4a
[...]   Match any character from set. Ranges like [a-z] are supported
[^...]  Match any character but ones from set

Under development: Unicode support.

API

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

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() can capture the respective substrings into the array of struct slre_cap structures:

/* Stores matched fragment for the expression inside brackets */
struct slre_cap {
  const char *ptr;  /* Points to the matched fragment */
  int len;          /* Length of the matched fragment */
};

N-th member of the caps array will contain fragment that corresponds to the N-th opening bracket in the regex, N is zero-based. slre_match() returns number of bytes scanned from the beginning of the string. If return value is greater or equal to 0, there is a match. If return value is less then 0, there is no match. Negative return codes are as follows:

#define SLRE_NO_MATCH               -1
#define SLRE_UNEXPECTED_QUANTIFIER  -2
#define SLRE_UNBALANCED_BRACKETS    -3
#define SLRE_INTERNAL_ERROR         -4
#define SLRE_INVALID_CHARACTER_SET  -5
#define SLRE_INVALID_METACHARACTER  -6
#define SLRE_CAPS_ARRAY_TOO_SMALL   -7
#define SLRE_TOO_MANY_BRANCHES      -8
#define SLRE_TOO_MANY_BRACKETS      -9

Example: parsing HTTP request line

const char *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, 4) > 0) {
  printf("Method: [%.*s], URI: [%.*s]\n",
         caps[0].len, caps[0].ptr,
         caps[1].len, caps[1].ptr);
} else {
  printf("Error parsing [%s]\n", request);
}

Example: find all URLs in a string

static const char *str =
  "<img src=\"HTTPS://FOO.COM/x?b#c=tab1\"/> "
  "  <a href=\"http://cesanta.com\">some link</a>";

static const char *regex = "(?i)((https?://)[^\\s/'\"<>]+/?[^\\s'\"<>]*)";
struct slre_cap caps[2];
int i, j = 0, str_len = strlen(str);

while (j < str_len &&
       (i = slre_match(regex, str + j, str_len - j, caps, 2, NULL)) > 0) {
  printf("Found URL: [%.*s]\n", caps[0].len, caps[0].ptr);
  j += i;
}

Output:

Found URL: [HTTPS://FOO.COM/x?b#c=tab1]
Found URL: [http://cesanta.com]

Licensing

SLRE is dual licensed. It is available either under the terms of GNU GPL v.2 license for free, or under the terms of standard commercial license provided by Cesanta Software. Businesses who whish to use Cesanta's products must license commercial version.

Super Light DNS Resolver, Mongoose web server are other projects by Cesanta Software, developed with the same philosophy of functionality and simplicity.