SLRE - Super Light RegEx version: master


SLRE is an ISO C library that implements a subset of Perl regular expression syntax. The 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 a simple intuitive API
  • Implements the 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.

^       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
\n      Match new line character
\r      Match line feed character
\f      Match form feed character
\v      Match vertical tab character
\t      Match horizontal tab character
\b      Match backspace character
+       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.

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

slre_match() matches the string buffer buf in length buf_len against the regular expression regexp, which should conform the syntax outlined above. If the 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 the number of bytes scanned from the beginning of the string. If the return value is greater or equal to 0, there is a match. If the return value is less then 0, there is no match. Negative return codes are as follows:

#define SLRE_NO_MATCH               -1
#define SLRE_INTERNAL_ERROR         -4
#define SLRE_TOO_MANY_BRANCHES      -8
#define SLRE_TOO_MANY_BRACKETS      -9

Valid flags are:

  • SLRE_IGNORE_CASE: do case-insensitive match

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) > 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);

Find all URLs in a string

static const char *str =
  " "
  "  some link";

static const char *regex = "((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, SLRE_IGNORE_CASE)) > 0) {
  printf("Found URL: [%.*s]\n", caps[0].len, caps[0].ptr);
  j += i;


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

SLRE is released under commercial and GNU GPL v.2 open source licenses.

Commercial Projects: Once your project becomes commercialised GPLv2 licensing dictates that you need to either open your source fully or purchase a commercial license. Cesanta offers full, royalty-free commercial licenses without any GPL restrictions. If your needs require a custom license, we’d be happy to work on a solution with you. Contact us for pricing.

Prototyping: While your project is still in prototyping stage and not for sale, you can use SLRE’s open source code without any license restrictions.