diff --git a/src/markup.c b/src/markup.c
index dba630f..884fb24 100644
--- a/src/markup.c
+++ b/src/markup.c
@@ -12,9 +12,14 @@
 #include "settings.h"
 #include "utils.h"
 
+/**
+ * Convert all HTML special symbols to HTML entities.
+ * @param str (nullable)
+ */
 static char *markup_quote(char *str)
 {
-        assert(str);
+        if (!str)
+                return NULL;
 
         str = string_replace_all("&", "&", str);
         str = string_replace_all("\"", """, str);
@@ -25,9 +30,14 @@ static char *markup_quote(char *str)
         return str;
 }
 
+/**
+ * Convert all HTML special entities to their actual char.
+ * @param str (nullable)
+ */
 static char *markup_unquote(char *str)
 {
-        assert(str);
+        if (!str)
+                return NULL;
 
         str = string_replace_all(""", "\"", str);
         str = string_replace_all("'", "'", str);
@@ -38,9 +48,14 @@ static char *markup_unquote(char *str)
         return str;
 }
 
+/**
+ * Convert all HTML linebreak tags to a newline character
+ * @param str (nullable)
+ */
 static char *markup_br2nl(char *str)
 {
-        assert(str);
+        if (!str)
+                return NULL;
 
         str = string_replace_all("<br>", "\n", str);
         str = string_replace_all("<br/>", "\n", str);
@@ -48,15 +63,10 @@ static char *markup_br2nl(char *str)
         return str;
 }
 
-/*
- * Remove HTML hyperlinks of a string.
- *
- * @str: The string to replace a tags
- * @urls: (nullable): If any href-attributes found, an '\n' concatenated
- *        string of the URLs in format '[<text between tags>] <href>'
- */
+/* see markup.h */
 void markup_strip_a(char **str, char **urls)
 {
+        assert(*str);
         char *tag1 = NULL;
 
         if (urls)
@@ -128,13 +138,7 @@ void markup_strip_a(char **str, char **urls)
         }
 }
 
-/*
- * Remove img-tags of a string. If alt attribute given, use this as replacement.
- *
- * @str: The string to replace img tags
- * @urls: (nullable): If any src-attributes found, an '\n' concatenated string of
- *        the URLs in format '[<alt>] <src>'
- */
+/* see markup.h */
 void markup_strip_img(char **str, char **urls)
 {
         const char *start;
@@ -217,18 +221,7 @@ void markup_strip_img(char **str, char **urls)
         }
 }
 
-/*
- * Strip any markup from text; turn it in to plain text.
- *
- * For well-formed markup, the following two commands should be
- * roughly equivalent:
- *
- *     out = markup_strip(in);
- *     pango_parse_markup(in, -1, 0, NULL, &out, NULL, NULL);
- *
- * However, `pango_parse_markup()` balks at invalid markup;
- * `markup_strip()` shouldn't care if there is invalid markup.
- */
+/* see markup.h */
 char *markup_strip(char *str)
 {
         if (!str)
@@ -317,10 +310,7 @@ static char *markup_escape_unsupported(char *str)
         return str;
 }
 
-/*
- * Transform the string in accordance with `markup_mode` and
- * `settings.ignore_newline`
- */
+/* see markup.h */
 char *markup_transform(char *str, enum markup_mode markup_mode)
 {
         if (!str)
diff --git a/src/markup.h b/src/markup.h
index 9d5cda7..aea4f0f 100644
--- a/src/markup.h
+++ b/src/markup.h
@@ -4,11 +4,42 @@
 
 #include "settings.h"
 
+/**
+ * Strip any markup from text; turn it in to plain text.
+ *
+ * For well-formed markup, the following two commands should be
+ * roughly equivalent:
+ *
+ *     out = markup_strip(in);
+ *     pango_parse_markup(in, -1, 0, NULL, &out, NULL, NULL);
+ *
+ * However, `pango_parse_markup()` balks at invalid markup;
+ * `markup_strip()` shouldn't care if there is invalid markup.
+ */
 char *markup_strip(char *str);
 
+/**
+ * Remove HTML hyperlinks of a string.
+ *
+ * @param str The string to replace a tags
+ * @param urls (nullable) If any href-attributes found, an `\n` concatenated
+ *        string of the URLs in format `[<text between tags>] <href>`
+ */
 void markup_strip_a(char **str, char **urls);
+
+/**
+ * Remove img-tags of a string. If alt attribute given, use this as replacement.
+ *
+ * @param str The string to replace img tags
+ * @param urls (nullable) If any src-attributes found, an `\n` concatenated string of
+ *        the URLs in format `[<alt>] <src>`
+ */
 void markup_strip_img(char **str, char **urls);
 
+/**
+ * Transform the string in accordance with `markup_mode` and
+ * `settings.ignore_newline`
+ */
 char *markup_transform(char *str, enum markup_mode markup_mode);
 
 #endif
diff --git a/src/utils.c b/src/utils.c
index adfa14b..22a0c50 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -2,6 +2,7 @@
 #include "utils.h"
 
 #include <assert.h>
+#include <ctype.h>
 #include <errno.h>
 #include <glib.h>
 #include <stdio.h>
@@ -10,16 +11,24 @@
 
 #include "log.h"
 
+/* see utils.h */
 char *string_replace_char(char needle, char replacement, char *haystack)
 {
+        if (!haystack)
+                return NULL;
+
         char *current = haystack;
         while ((current = strchr(current, needle)))
                 *current++ = replacement;
         return haystack;
 }
 
+/* see utils.h */
 char *string_replace_at(char *buf, int pos, int len, const char *repl)
 {
+        assert(buf);
+        assert(repl);
+
         char *tmp;
         int size, buf_len, repl_len;
 
@@ -44,18 +53,14 @@ char *string_replace_at(char *buf, int pos, int len, const char *repl)
         return tmp;
 }
 
-char *string_replace(const char *needle, const char *replacement, char *haystack)
-{
-        char *start;
-        start = strstr(haystack, needle);
-        if (!start)
-                return haystack;
-
-        return string_replace_at(haystack, (start - haystack), strlen(needle), replacement);
-}
-
+/* see utils.h */
 char *string_replace_all(const char *needle, const char *replacement, char *haystack)
 {
+        if (!haystack)
+                return NULL;
+        assert(needle);
+        assert(replacement);
+
         char *start;
         int needle_pos;
         int needle_len, repl_len;
@@ -76,6 +81,7 @@ char *string_replace_all(const char *needle, const char *replacement, char *hays
         return haystack;
 }
 
+/* see utils.h */
 char *string_append(char *a, const char *b, const char *sep)
 {
         if (STR_EMPTY(a)) {
@@ -93,7 +99,6 @@ char *string_append(char *a, const char *b, const char *sep)
         g_free(a);
 
         return new;
-
 }
 
 /* see utils.h */
@@ -113,8 +118,11 @@ char *string_strip_quotes(const char *value)
         return s;
 }
 
+/* see utils.h */
 void string_strip_delimited(char *str, char a, char b)
 {
+        assert(str);
+
         int iread=-1, iwrite=0, copen=0;
         while (str[++iread] != 0) {
                 if (str[iread] == a) {
@@ -128,13 +136,14 @@ void string_strip_delimited(char *str, char a, char b)
         str[iwrite] = 0;
 }
 
+/* see utils.h */
 char *string_to_path(char *string)
 {
 
         if (string && STRN_EQ(string, "~/", 2)) {
                 char *home = g_strconcat(getenv("HOME"), "/", NULL);
 
-                string = string_replace("~/", home, string);
+                string = string_replace_at(string, 0, 2, home);
 
                 g_free(home);
         }
@@ -142,9 +151,9 @@ char *string_to_path(char *string)
         return string;
 }
 
+/* see utils.h */
 gint64 string_to_time(const char *string)
 {
-
         assert(string);
 
         errno = 0;
@@ -165,7 +174,7 @@ gint64 string_to_time(const char *string)
         }
 
         // endptr may point to a separating space
-        while (*endptr == ' ')
+        while (isspace(*endptr))
                 endptr++;
 
         if (STRN_EQ(endptr, "ms", 2))
diff --git a/src/utils.h b/src/utils.h
index 6ea3b4e..b77fa28 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -16,23 +16,49 @@
 //! Test if string a and b are the same case-insensitively
 #define STR_CASEQ(a, b) (strcasecmp(a, b) == 0)
 
-/* replace all occurrences of the character needle with the character replacement in haystack */
+/**
+ * Replaces all occurrences of the char \p needle with the char \p replacement in \p haystack.
+ *
+ * Does not allocate a new string.
+ *
+ * @param needle The char to replace with replacement
+ * @param replacement The char which is the new one
+ * @param haystack (nullable) The string to replace
+ *
+ * @returns The exact value of the haystack paramater (to allow nesting)
+ */
 char *string_replace_char(char needle, char replacement, char *haystack);
 
-/* replace all occurrences of needle with replacement in haystack */
-char *string_replace_all(const char *needle, const char *replacement, char *haystack);
-
-/* replace <len> characters with <repl> at position <pos> of the string <buf> */
+/**
+ * Replace a substring inside a string with another string.
+ *
+ * May reallocate memory. Free the result with `g_free`.
+ *
+ * @param buf The string to replace
+ * @param pos The position of the substring to replace
+ * @param len The length of the substring to replace
+ * @param repl The new contents of the substring.
+ */
 char *string_replace_at(char *buf, int pos, int len, const char *repl);
 
-/* replace needle with replacement in haystack */
-char *string_replace(const char *needle, const char *replacement, char *haystack);
+/**
+ * Replace all occurences of a substring.
+ *
+ * @param needle The substring to search
+ * @param replacement The substring to replace
+ * @param haystack (nullable) The string to search the substring for
+ */
+char *string_replace_all(const char *needle, const char *replacement, char *haystack);
 
+/**
+ * Append \p b to string \p a. And concatenate both strings with \p concat, if both are non-empty.
+ *
+ * @param a (nullable) The left side of the string
+ * @param b (nullable) The right side of the string
+ * @param sep (nullable) The concatenator to concatenate if a and b given
+ */
 char *string_append(char *a, const char *b, const char *sep);
 
-/* strip content between two delimiter characters (inplace) */
-void string_strip_delimited(char *str, char a, char b);
-
 /**
  * Strip quotes from a string. Won't touch inner quotes.
  *
@@ -41,10 +67,32 @@ void string_strip_delimited(char *str, char a, char b);
  */
 char *string_strip_quotes(const char *value);
 
-/* replace tilde and path-specific values with its equivalents */
+/**
+ * Strip content between two delimiter characters
+ *
+ * @param str The string to operate in place
+ * @param a Starting delimiter
+ * @param b Ending delimiter
+ */
+void string_strip_delimited(char *str, char a, char b);
+
+/**
+ * Replace tilde and path-specific values with its equivalents
+ *
+ * The string gets invalidated. The new valid representation is the return value.
+ *
+ * @param string (nullable) The string to convert to a path.
+ * @returns The tilde-replaced string.
+ */
 char *string_to_path(char *string);
 
-/* convert time units (ms, s, m) to internal gint64 microseconds */
+/**
+ * Convert time units (ms, s, m) to the internal `gint64` microseconds format
+ *
+ * If no unit is given, 's' (seconds) is assumed by default.
+ *
+ * @param string The string to parse the time format from.
+ */
 gint64 string_to_time(const char *string);
 
 /**
diff --git a/test/utils.c b/test/utils.c
index b99af9e..91e99fb 100644
--- a/test/utils.c
+++ b/test/utils.c
@@ -5,7 +5,8 @@
 
 TEST test_string_replace_char(void)
 {
-        char *text = malloc(128 * sizeof(char));
+        char *text = g_malloc(128 * sizeof(char));
+
         strcpy(text, "a aa aaa");
         ASSERT_STR_EQ("b bb bbb", string_replace_char('a', 'b', text));
 
@@ -14,8 +15,8 @@ TEST test_string_replace_char(void)
 
         strcpy(text, "");
         ASSERT_STR_EQ("", string_replace_char('a', 'b', text));
-        free(text);
 
+        g_free(text);
         PASS();
 }
 
@@ -27,8 +28,8 @@ TEST test_string_replace_char(void)
 
 TEST test_string_replace_all(void)
 {
+        char *text = g_malloc(128 * sizeof(char));
 
-        char *text = malloc(128 * sizeof(char));
         strcpy(text, "aaaaa");
         ASSERT_STR_EQ("bbbbb", (text = string_replace_all("a", "b", text)));
 
@@ -44,29 +45,7 @@ TEST test_string_replace_all(void)
         strcpy(text, "abcdabc");
         ASSERT_STR_EQ("xyzabcdxyzabc", (text = string_replace_all("a", "xyza", text)));
 
-        free(text);
-        PASS();
-}
-
-TEST test_string_replace(void)
-{
-        char *text = malloc(128 * sizeof(char));
-        strcpy(text, "aaaaa");
-        ASSERT_STR_EQ("baaaa", (text = string_replace("a", "b", text)) );
-
-        strcpy(text, "");
-        ASSERT_STR_EQ((text = string_replace("a", "b", text)), "");
-
-        strcpy(text, "Nothing to replace");
-        ASSERT_STR_EQ((text = string_replace("z", "a", text)), "Nothing to replace");
-
-        strcpy(text, "Reverse this");
-        ASSERT_STR_EQ("Reverse sith", (text = string_replace("this", "sith", text)));
-
-        strcpy(text, "abcdabc");
-        ASSERT_STR_EQ("xyzabcdabc", (text = string_replace("a", "xyza", text)));
-
-        free(text);
+        g_free(text);
         PASS();
 }
 
@@ -132,7 +111,7 @@ TEST test_string_strip_quotes(void)
 
 TEST test_string_strip_delimited(void)
 {
-        char *text = malloc(128 * sizeof(char));
+        char *text = g_malloc(128 * sizeof(char));
 
         strcpy(text, "A <simple> string_strip_delimited test");
         string_strip_delimited(text, '<', '>');
@@ -154,7 +133,7 @@ TEST test_string_strip_delimited(void)
         string_strip_delimited(text, '<', '>');
         ASSERT_STR_EQ("Nothing is done if there are no delimiters in the string", text);
 
-        free(text);
+        g_free(text);
         PASS();
 }
 
@@ -202,7 +181,6 @@ SUITE(suite_utils)
 {
         RUN_TEST(test_string_replace_char);
         RUN_TEST(test_string_replace_all);
-        RUN_TEST(test_string_replace);
         RUN_TEST(test_string_append);
         RUN_TEST(test_string_strip_quotes);
         RUN_TEST(test_string_strip_delimited);