diff --git a/include/pattern.h b/include/pattern.h
index 70b1d1e..c267c0c 100644
--- a/include/pattern.h
+++ b/include/pattern.h
@@ -170,7 +170,7 @@ typedef struct pattern_s pattern;
  * @param id the id of the pattern.
  * @return LIBLEX_SUCCESS if all goes well, otherwise some other liblex_result.
  */
-liblex_result pattern_compile(pattern* ptrn, char* expression, int id);
+liblex_result pattern_compile(pattern* ptrn, const char* expression, int id);
 /**
  * Frees a pattern NFA allocated by pattern_compile.
  * @param root the root node to start freeing from.
diff --git a/src/pattern.c b/src/pattern.c
index 125c08e..cea11f7 100644
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -172,7 +172,7 @@ liblex_result _pattern_free(pattern_node* root, int size){
   sprs set;
   liblex_result result;
   sprs_init(&set);
-  result = (sprs_setup(&set, size) == LIBDS_SUCCESS) ? LIBLEX_SUCCESS : LIBLEX_MALLOC;
+  result = (sprs_setup(&set, (size_t) size) == LIBDS_SUCCESS) ? LIBLEX_SUCCESS : LIBLEX_MALLOC;
   if(result == LIBLEX_SUCCESS){
     _pattern_node_find_all(root, &set);
     sprs_foreach(&set, NULL, compare_always, _pattern_node_foreach_free);
@@ -181,7 +181,7 @@ liblex_result _pattern_free(pattern_node* root, int size){
   return result;
 }
 
-liblex_result _pattern_read_value(char* read_into, char* string, int* index){
+liblex_result _pattern_read_value(char* read_into, const char* string, int* index){
   liblex_result result = LIBLEX_SUCCESS;
   if(string[*index] == '\\'){
     (*index)++;
@@ -195,7 +195,7 @@ liblex_result _pattern_read_value(char* read_into, char* string, int* index){
   return result;
 }
 
-liblex_result _pattern_build_or(pattern_chain** into, int* ids, int pattern_id, char* string, int* index) {
+liblex_result _pattern_build_or(pattern_chain** into, int* ids, int pattern_id, const char* string, int* index) {
   pattern_node *tail_node;
   liblex_result result = LIBLEX_SUCCESS;
   result = _pattern_node_create_connect(&tail_node, (*ids)++, pattern_id, NULL);
@@ -265,7 +265,7 @@ void _pattern_chain_append_chain_discard(pattern_chain* append_to, pattern_chain
   *to_append = NULL;
 }
 
-liblex_result _pattern_build_chain(pattern_chain** into, int* ids, int pattern_id, char* string, int* index){
+liblex_result _pattern_build_chain(pattern_chain** into, int* ids, int pattern_id, const char* string, int* index){
   liblex_result result = LIBLEX_SUCCESS;
   ll or_stack;
   pattern_chain* current_chain = NULL;
@@ -421,7 +421,7 @@ liblex_result _pattern_build_chain(pattern_chain** into, int* ids, int pattern_i
   return result;
 }
 
-liblex_result pattern_compile(pattern* ptrn, char* expression, int id){
+liblex_result pattern_compile(pattern* ptrn, const char* expression, int id){
   liblex_result result;
   pattern_node* end_node = NULL;
   pattern_chain* full_chain = NULL;