diff --git a/bootstrap b/bootstrap
index 3352589..751cc7c 100755
--- a/bootstrap
+++ b/bootstrap
@@ -1,16 +1,14 @@
 #!/bin/sh -e
 
-if [ `uname -s` = "Darwin" ]; then
-echo "WARNING: OSX autogen build is not supported"
-fi
-
 REQUIRED='is required, but not installed.'
 bc -v >/dev/null 2>&1 || { echo >&2 "'bc' $REQUIRED"; exit 1; }
-if [ `uname -s` != "FreeBSD" ]; then
-sed --version >/dev/null 2>&1 || { echo >&2 "'sed' $REQUIRED"; exit 1; }
-fi
+[ "x`echo hello | sed s/hello/world/ 2>/dev/null`" = "xworld" ] || { echo >&2 "'sed' $REQUIRED"; exit 1; }
 autoreconf --version >/dev/null 2>&1 || { echo >&2 "'autoconf' $REQUIRED"; exit 1; }
 
+# Determine which flag sed uses for extended regular expressions.
+# -E is POSIX. -r is for GNU sed older than 4.2.
+echo hello | sed -E s/hello/world/ >/dev/null 2>&1 && SED_ERE=-E || SED_ERE=-r
+
 # If libtool is not installed -> "error: Libtool library used but 'LIBTOOL' is undefined"
 
 if [ ! -e "./m4" ]; then
@@ -31,7 +29,7 @@
 BROTLI_VERSION_PATCH=`echo "$BROTLI_VERSION_INT % 4096" | bc`
 BROTLI_VERSION="$BROTLI_VERSION_MAJOR.$BROTLI_VERSION_MINOR.$BROTLI_VERSION_PATCH"
 
-sed -i.bak -r "s/[0-9]+:[0-9]+:[0-9]+/$BROTLI_ABI_INFO/" Makefile.am
-sed -i.bak -r "s/\[[0-9]+\.[0-9]+\.[0-9]+\]/[$BROTLI_VERSION]/" configure.ac
+sed -i.bak "$SED_ERE" "s/[0-9]+:[0-9]+:[0-9]+/$BROTLI_ABI_INFO/" Makefile.am
+sed -i.bak "$SED_ERE" "s/\[[0-9]+\.[0-9]+\.[0-9]+\]/[$BROTLI_VERSION]/" configure.ac
 
 autoreconf --install --force --symlink || exit $