Use extended regex instead of Perl regex

package.sh

@@ -2,15 +2,17 @@
 #
 set -e
 
+# Shorthand for digit class:
+d="[[:digit:]]"
 # Determine the version and build timestamp
 VERSION=$(sed -n -e "s/^set(log4cxx_VER \"\(.*\)\")/\1/p" < src/cmake/projectVersionDetails.cmake)
-if ! echo "$VERSION" | grep -Pq '^\d+\.\d+\.\d+$'; then
+if ! echo "$VERSION" | grep -Eq "^$d+\.$d+\.$d+$"; then
   echo Invalid version number: "$VERSION" >& 2
   exit 1
 fi
 
 OUTPUT_TIMESTAMP=$(sed -n -e "s/^set(log4cxx_OUTPUT_TIMESTAMP \"\(.*\)\")/\1/p" < src/cmake/projectVersionDetails.cmake)
-if ! echo "$OUTPUT_TIMESTAMP" | grep -Pq '^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z$'; then
+if ! echo "$OUTPUT_TIMESTAMP" | grep -Eq "^$d{4}-$d{2}-$d{2}T$d{2}:$d{2}:$d{2}Z$"; then
   echo Invalid build timestamp: "$OUTPUT_TIMESTAMP" >& 2
   echo Run '`'date -u +%Y-%m-%dT%H:%M:%SZ'`' to generate it
   exit 1