diff --git a/src/FileCheck b/src/FileCheck
index 7ef1819..e88cc1d 100755
--- a/src/FileCheck
+++ b/src/FileCheck
@@ -106,7 +106,8 @@ with open(check_file) as f:
                 match_type = MatchType.REGEX
                 check_expression = regex_line
             else:
-                check_expression = re.sub("\\s+", ' ', check_expression).strip()
+                if not args.strict_whitespace:
+                    check_expression = re.sub("\\s+", ' ', check_expression).strip()
 
             check = Check(check_type=CheckType.CHECK_NOT,
                           match_type=match_type,
@@ -175,7 +176,8 @@ for line_idx, line in enumerate(sys.stdin):
 
     elif current_check.check_type == CheckType.CHECK_NOT:
         if current_check.match_type == MatchType.SUBSTRING:
-            line = re.sub("\\s+", ' ', line).strip()
+            if not args.strict_whitespace:
+                line = re.sub("\\s+", ' ', line).strip()
 
             if current_check.expression in line:
                 break
@@ -243,6 +245,9 @@ if current_check.check_type == CheckType.CHECK_NOT:
     if (current_check.match_type == MatchType.SUBSTRING or
             current_check.match_type == MatchType.REGEX):
         last_read_line = input_lines[-1]
+        if not args.strict_whitespace:
+            last_read_line = re.sub("\\s+", ' ', last_read_line).strip()
+
         print("{}:{}:{}: error: CHECK-NOT: excluded string found in input"
               .format(check_file,
                       current_check.check_line_idx + 1,
diff --git a/tests/integration/tests/options/--strict-whitespace/01_input_has_mixed_spaces_and_tabs/filecheck.check b/tests/integration/tests/options/--strict-whitespace/CHECK/01_input_has_mixed_spaces_and_tabs/filecheck.check
similarity index 100%
rename from tests/integration/tests/options/--strict-whitespace/01_input_has_mixed_spaces_and_tabs/filecheck.check
rename to tests/integration/tests/options/--strict-whitespace/CHECK/01_input_has_mixed_spaces_and_tabs/filecheck.check
diff --git a/tests/integration/tests/options/--strict-whitespace/01_input_has_mixed_spaces_and_tabs/filecheck.input b/tests/integration/tests/options/--strict-whitespace/CHECK/01_input_has_mixed_spaces_and_tabs/filecheck.input
similarity index 100%
rename from tests/integration/tests/options/--strict-whitespace/01_input_has_mixed_spaces_and_tabs/filecheck.input
rename to tests/integration/tests/options/--strict-whitespace/CHECK/01_input_has_mixed_spaces_and_tabs/filecheck.input
diff --git a/tests/integration/tests/options/--strict-whitespace/01_input_has_mixed_spaces_and_tabs/sample.itest b/tests/integration/tests/options/--strict-whitespace/CHECK/01_input_has_mixed_spaces_and_tabs/sample.itest
similarity index 100%
rename from tests/integration/tests/options/--strict-whitespace/01_input_has_mixed_spaces_and_tabs/sample.itest
rename to tests/integration/tests/options/--strict-whitespace/CHECK/01_input_has_mixed_spaces_and_tabs/sample.itest
diff --git a/tests/integration/tests/options/--strict-whitespace/02_check_has_mixed_spaces_and_tabs/filecheck.check b/tests/integration/tests/options/--strict-whitespace/CHECK/02_check_has_mixed_spaces_and_tabs/filecheck.check
similarity index 100%
rename from tests/integration/tests/options/--strict-whitespace/02_check_has_mixed_spaces_and_tabs/filecheck.check
rename to tests/integration/tests/options/--strict-whitespace/CHECK/02_check_has_mixed_spaces_and_tabs/filecheck.check
diff --git a/tests/integration/tests/options/--strict-whitespace/02_check_has_mixed_spaces_and_tabs/filecheck.input b/tests/integration/tests/options/--strict-whitespace/CHECK/02_check_has_mixed_spaces_and_tabs/filecheck.input
similarity index 100%
rename from tests/integration/tests/options/--strict-whitespace/02_check_has_mixed_spaces_and_tabs/filecheck.input
rename to tests/integration/tests/options/--strict-whitespace/CHECK/02_check_has_mixed_spaces_and_tabs/filecheck.input
diff --git a/tests/integration/tests/options/--strict-whitespace/02_check_has_mixed_spaces_and_tabs/sample.itest b/tests/integration/tests/options/--strict-whitespace/CHECK/02_check_has_mixed_spaces_and_tabs/sample.itest
similarity index 100%
rename from tests/integration/tests/options/--strict-whitespace/02_check_has_mixed_spaces_and_tabs/sample.itest
rename to tests/integration/tests/options/--strict-whitespace/CHECK/02_check_has_mixed_spaces_and_tabs/sample.itest
diff --git a/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/filecheck.check b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/filecheck.check
new file mode 100644
index 0000000..a43cced
--- /dev/null
+++ b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/filecheck.check
@@ -0,0 +1 @@
+; CHECK-NOT: hello world hello world
diff --git a/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/filecheck.input b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/filecheck.input
new file mode 100644
index 0000000..2e126aa
--- /dev/null
+++ b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/filecheck.input
@@ -0,0 +1 @@
+hello   world hello  world
diff --git a/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/sample.itest b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/sample.itest
new file mode 100644
index 0000000..5444cee
--- /dev/null
+++ b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/01_input_has_mixed_spaces_and_tabs/sample.itest
@@ -0,0 +1,14 @@
+; RUN: cat %S/filecheck.input | (%FILECHECK_EXEC %S/filecheck.check 2>&1; test $? = 1) | %FILECHECK_TESTER_EXEC %s --strict-whitespace --match-full-lines -check-prefix=NO_OPTION
+; RUN: cat %S/filecheck.input | (%FILECHECK_EXEC %S/filecheck.check --strict-whitespace 2>&1; test $? = 0) | %FILECHECK_TESTER_EXEC %s --match-full-lines -check-prefix=OPTION
+
+; NO_OPTION:{{^.*}}FileCheck{{$}}
+; NO_OPTION:{{^.*}}filecheck.check:1:14: error: CHECK-NOT: excluded string found in input{{$}}
+; NO_OPTION:; CHECK-NOT: hello world hello world
+; NO_OPTION:             ^
+; NO_OPTION:{{^<stdin>:.:.: note: found here$}}
+; NO_OPTION:hello world hello world
+; NO_OPTION:^~~~~~~~~~~~~~~~~~~~~~~
+; NO_OPTION-EMPTY:
+
+; OPTION: {{^.*}}FileCheck{{$}}
+; OPTION-EMPTY:
diff --git a/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/filecheck.check b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/filecheck.check
new file mode 100644
index 0000000..0b8daea
--- /dev/null
+++ b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/filecheck.check
@@ -0,0 +1 @@
+; CHECK: hello   world hello  world
diff --git a/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/filecheck.input b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/filecheck.input
new file mode 100644
index 0000000..235735d
--- /dev/null
+++ b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/filecheck.input
@@ -0,0 +1 @@
+hello world hello world
diff --git a/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/sample.itest b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/sample.itest
new file mode 100644
index 0000000..da30e46
--- /dev/null
+++ b/tests/integration/tests/options/--strict-whitespace/CHECK_NOT/02_check_has_mixed_spaces_and_tabs/sample.itest
@@ -0,0 +1,15 @@
+; RUN: cat %S/filecheck.input | %FILECHECK_EXEC %S/filecheck.check | %FILECHECK_TESTER_EXEC %s --match-full-lines -check-prefix=NO_OPTION
+; RUN: cat %S/filecheck.input | (%FILECHECK_EXEC %S/filecheck.check --strict-whitespace 2>&1; test $? = 1) | %FILECHECK_TESTER_EXEC %s --strict-whitespace --match-full-lines -check-prefix=OPTION
+
+; NO_OPTION: {{^.*}}FileCheck{{$}}
+; NO_OPTION-EMPTY:
+
+; OPTION:{{^.*}}FileCheck{{$}}
+; OPTION:{{^.*}}filecheck.check:1:10: error: CHECK: expected string not found in input{{$}}
+; OPTION:; CHECK: hello   world hello  world
+; OPTION:         ^
+; OPTION:{{^<stdin>:.*:.*: note: scanning from here$}}
+; OPTION:hello world hello world
+; OPTION:^
+; TODO: error: OPTION-EMPTY is not on the line after the previous match
+; OPTION-EMPTY