regex_escape: support POSIX basic regex (#50327)

changelogs/fragments/regex-escape-basic.yaml

@@ -0,0 +1,8 @@
+minor_changes:
+- |
+    regex_escape - added re_type option to enable escaping POSIX BRE chars
+
+    This distinction is necessary because escaping non-special chars such as
+    '(' or '{' turns them into special chars, the opposite of what is intended
+    by using regex_escape on strings being passed as a Basic Regular
+    Expression.

docs/docsite/rst/user_guide/playbooks_filters.rst

@@ -1075,11 +1075,18 @@ To replace text in a string with regex, use the "regex_replace" filter::
 
 .. versionadded:: 2.0
 
-To escape special characters within a regex, use the "regex_escape" filter::
+To escape special characters within a standard python regex, use the "regex_escape" filter (using the default re_type='python' option)::
 
     # convert '^f.*o(.*)$' to '\^f\.\*o\(\.\*\)\$'
     {{ '^f.*o(.*)$' | regex_escape() }}
 
+.. versionadded:: 2.8
+
+To escape special characters within a POSIX basic regex, use the "regex_escape" filter with the re_type='posix_basic' option::
+
+    # convert '^f.*o(.*)$' to '\^f\.\*o(\.\*)\$'
+    {{ '^f.*o(.*)$' | regex_escape('posix_basic') }}
+
 
 Kubernetes Filters
 ``````````````````

lib/ansible/plugins/filter/core.py

@@ -185,9 +185,22 @@ def ternary(value, true_val, false_val, none_val=None):
         return false_val
 
 
-def regex_escape(string):
+def regex_escape(string, re_type='python'):
     '''Escape all regular expressions special characters from STRING.'''
-    return re.escape(string)
+    if re_type == 'python':
+        return re.escape(string)
+    elif re_type == 'posix_basic':
+        # list of BRE special chars:
+        # https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions
+        return regex_replace(string, r'([].[^$*\\])', r'\\\1')
+    # TODO: implement posix_extended
+    # It's similar to, but different from python regex, which is similar to,
+    # but different from PCRE.  It's possible that re.escape would work here.
+    # https://remram44.github.io/regex-cheatsheet/regex.html#programs
+    elif re_type == 'posix_extended':
+        raise AnsibleFilterError('Regex type (%s) not yet implemented' % re_type)
+    else:
+        raise AnsibleFilterError('Invalid regex type (%s)' % re_type)
 
 
 def from_yaml(data):