Create README.md

README.md

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+# liblex
+
+A library for converting input text into tokens defined by regular expressions.
+
+## Rationale
+
+liblex is a part of an attempt to write / create a compiler entirely from scratch.
+This part of the compiler would be used to convert input text into tokens
+to be evaluated by the parser.
+
+## Usage
+
+First of all, an evulation configuration has to be created. This configuration
+is used to store the various regular expressions to be used during
+lexing. The below code is an example of initializing and configuring an
+evalutation configuration.
+
+```C
+/* Declares the configuration */
+eval_config config;
+/* Initializes the configuration for use */
+eval_config_init(&config);
+/* Registers regular expressions to be used. The IDs, given as the third
+   parameter are also used for priority - the higher the ID, the higher the
+   priority. */
+eval_config_add(&config, "[ \n]+", 0);
+eval_config_add(&config, "[a-zA-Z_][a-zA-Z_0-9]*", 1);
+eval_config_add(&config, "if", 2);
+eval_config_add(&config, "else", 3);
+eval_config_add(&config, "[0-9]+", 4);
+eval_config_add(&config, "{|}", 5);
+```
+
+It should be noted that this example is incomplete. `eval_config_add` returns
+a `liblex_result`, which represents the result of the operation. `LIBLEX_SUCCESS`
+means that no errors occured. `LIBLEX_MALLOC`, on the other hand, means that
+the function failed to allocate the necessary memory, and `LIBLEX_INVALID`
+means that the regular expression provided was not correctly formatted.
+
+After the eval configuration has been configured, tokenizing a string is
+done by creating a linked list and populating it with the resulting tokens
+(called matches). 
+
+```
+/* Declares the linked list. */
+ll match_ll;
+/* Initializes the linked list. */
+ll_init(&match_ll);
+
+/* The first parameter is the input string, the second is the index at which
+   to begin parsing. */
+eval_all(string, 0, &config, &match_ll);
+```
+
+Once done, some things need to be cleaned up. The `eval_foreach_match_free`
+function can be passed to `ll_foreach` containing the matches to release them:
+```
+ll_foreach(&match_ll, NULL, compare_always, eval_foreach_match_free);
+ll_clear(&match_ll);
+```
+And the configuration can be freed using:
+```
+eval_config_free(&config);
+```