Changed to allow specifying engine
Optional argument added to maintain backward compatibility

Signed-off-by: David Bibby <davidsbibby@gmail.com>

README.md

diff --git a/README.md b/README.md
index 48fe5b5..86c56b0 100644
--- a/README.md
+++ b/README.md
@@ -739,9 +739,11 @@
 Fail if the given file does not contain the regex.
 ```bash
 @test 'assert_file_contains() {
-    assert_file_contains /path/to/non-empty-file regex
+    assert_file_contains /path/to/non-empty-file regex engine
 }
 ```
+`engine` is optional and can be one of `grep`, `egrep` or `pcregrep`. The specified engine must be available on the system running the tests.
+
 On failure, the path and expected regex are displayed.
 
 [Back to index](#Index-of-all-functions)

src/file.bash

diff --git a/src/file.bash b/src/file.bash
index 5fe0ef5..8eee540 100644
--- a/src/file.bash
+++ b/src/file.bash
@@ -543,6 +543,7 @@
 # Arguments:
 #   $1 - path
 #   $2 - regex
+#   $3 - grep engine to use (grep, egrep, pcregrep) - optional
 # Returns:
 #   0 - file contains regex
 #   1 - otherwise
@@ -551,7 +552,13 @@
 assert_file_contains() {
   local -r file="$1"
   local -r regex="$2"
-  if ! grep -E -q "$regex" "$file"; then
+  local -r cmd="${3:-grep}"
+  local -r engines=(grep egrep pcregrep)
+  if [ -z "$(echo ${engines[@]} | grep -w ${cmd})" ] || ! which ${cmd} > /dev/null 2>&1; then
+    batslib_decorate "Regex engine \"${cmd}\" not available on this system" \
+      | fail
+  fi
+  if ! $cmd -q "$regex" "$file"; then
     local -r rem="${BATSLIB_FILE_PATH_REM-}"
     local -r add="${BATSLIB_FILE_PATH_ADD-}"
     batslib_print_kv_single 4 'path' "${file/$rem/$add}" \