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diff --git a/src/string_utils.h b/src/string_utils.h
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+++ b/src/string_utils.h
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+/*
+  Copyright 2011-2020 David Robillard <http://drobilla.net>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THIS SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#ifndef SERD_STRING_UTILS_H
+#define SERD_STRING_UTILS_H
+
+#include "serd/serd.h"
+
+#include <assert.h>
+#include <ctype.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/** Unicode replacement character in UTF-8 */
+static const uint8_t replacement_char[] = { 0xEF, 0xBF, 0xBD };
+
+/** Return true if `c` lies within [`min`...`max`] (inclusive) */
+static inline bool
+in_range(const int c, const int min, const int max)
+{
+	return (c >= min && c <= max);
+}
+
+/** RFC2234: ALPHA ::= %x41-5A / %x61-7A  ; A-Z / a-z */
+static inline bool
+is_alpha(const int c)
+{
+	return in_range(c, 'A', 'Z') || in_range(c, 'a', 'z');
+}
+
+/** RFC2234: DIGIT ::= %x30-39  ; 0-9 */
+static inline bool
+is_digit(const int c)
+{
+	return in_range(c, '0', '9');
+}
+
+/* RFC2234: HEXDIG ::= DIGIT / "A" / "B" / "C" / "D" / "E" / "F" */
+static inline bool
+is_hexdig(const int c)
+{
+	return is_digit(c) || in_range(c, 'A', 'F');
+}
+
+/* Turtle / JSON / C: XDIGIT ::= DIGIT / A-F / a-f */
+static inline bool
+is_xdigit(const int c)
+{
+	return is_hexdig(c) || in_range(c, 'a', 'f');
+}
+
+static inline bool
+is_space(const char c)
+{
+	switch (c) {
+	case ' ': case '\f': case '\n': case '\r': case '\t': case '\v':
+		return true;
+	default:
+		return false;
+	}
+}
+
+static inline bool
+is_base64(const uint8_t c)
+{
+	return is_alpha(c) || is_digit(c) || c == '+' || c == '/' || c == '=';
+}
+
+static inline bool
+is_windows_path(const uint8_t* path)
+{
+	return is_alpha(path[0]) && (path[1] == ':' || path[1] == '|')
+		&& (path[2] == '/' || path[2] == '\\');
+}
+
+size_t
+serd_substrlen(const uint8_t* str,
+               size_t         len,
+               size_t*        n_bytes,
+               SerdNodeFlags* flags);
+
+static inline int
+serd_strncasecmp(const char* s1, const char* s2, size_t n)
+{
+	for (; n > 0 && *s2; s1++, s2++, --n) {
+		if (toupper(*s1) != toupper(*s2)) {
+			return ((*(const uint8_t*)s1 < *(const uint8_t*)s2) ? -1 : +1);
+		}
+	}
+	return 0;
+}
+
+static inline uint32_t
+utf8_num_bytes(const uint8_t c)
+{
+	if ((c & 0x80) == 0) {  // Starts with `0'
+		return 1;
+	} else if ((c & 0xE0) == 0xC0) {  // Starts with `110'
+		return 2;
+	} else if ((c & 0xF0) == 0xE0) {  // Starts with `1110'
+		return 3;
+	} else if ((c & 0xF8) == 0xF0) {  // Starts with `11110'
+		return 4;
+	}
+	return 0;
+}
+
+/// Return the code point of a UTF-8 character with known length
+static inline uint32_t
+parse_counted_utf8_char(const uint8_t* utf8, size_t size)
+{
+	uint32_t c = utf8[0] & ((1u << (8 - size)) - 1);
+	for (size_t i = 1; i < size; ++i) {
+		const uint8_t in = utf8[i] & 0x3F;
+		c = (c << 6) | in;
+	}
+	return c;
+}
+
+/// Parse a UTF-8 character, set *size to the length, and return the code point
+static inline uint32_t
+parse_utf8_char(const uint8_t* utf8, size_t* size)
+{
+	switch (*size = utf8_num_bytes(utf8[0])) {
+	case 1: case 2: case 3: case 4:
+		return parse_counted_utf8_char(utf8, *size);
+	default:
+		*size = 0;
+		return 0;
+	}
+}
+
+#endif  // SERD_STRING_UTILS_H