diff --git a/src/calibre/ebooks/css_transform_rules.py b/src/calibre/ebooks/css_transform_rules.py
index 3bd8428c2d..acf8e4b44d 100644
--- a/src/calibre/ebooks/css_transform_rules.py
+++ b/src/calibre/ebooks/css_transform_rules.py
@@ -75,10 +75,13 @@ class StyleDeclaration(object):
         dec._setSeq(seq)
         self.changed = True
 
-    def change_property(self, prop, parent_prop, val):
+    def change_property(self, prop, parent_prop, val, match_pat=None):
         if parent_prop is not None:
             self.expand_property(parent_prop)
-        prop.value = val
+        if match_pat is None:
+            prop.value = val
+        else:
+            prop.value = match_pat.sub(val, prop.value)
         self.changed = True
 
     def append_properties(self, props):
@@ -167,6 +170,7 @@ class Rule(object):
     def __init__(self, property='color', match_type='*', query='', action='remove', action_data=''):
         self.property_name = property.lower()
         self.action, self.action_data = action, action_data
+        self.match_pat = None
         if self.action == 'append':
             decl = safe_parser().parseStyle(self.action_data)
             self.appended_properties = list(all_properties(decl))
@@ -179,11 +183,11 @@ class Rule(object):
         elif match_type == '*':
             self.property_matches = lambda x: True
         elif 'matches' in match_type:
-            q = compile_pat(query)
+            self.match_pat = compile_pat(query)
             if match_type.startswith('not_'):
-                self.property_matches = lambda x: q.match(x) is None
+                self.property_matches = lambda x: self.match_pat.match(x) is None
             else:
-                self.property_matches = lambda x: q.match(x) is not None
+                self.property_matches = lambda x: self.match_pat.match(x) is not None
         else:
             value, unit = parse_css_length_or_number(query)
             op = getattr(operator, operator_map[match_type])
@@ -197,7 +201,7 @@ class Rule(object):
                 if self.action == 'remove':
                     declaration.remove_property(prop, parent_prop)
                 elif self.action == 'change':
-                    declaration.change_property(prop, parent_prop, self.action_data)
+                    declaration.change_property(prop, parent_prop, self.action_data, self.match_pat)
                 elif self.action == 'append':
                     declaration.append_properties(self.appended_properties)
                 else:
@@ -385,8 +389,15 @@ def test(return_tests=False):  # {{{
 
         def test_matching(self):
 
-            def m(match_type='*', query=''):
-                self.ae(apply_rule(css, property=prop, match_type=match_type, query=query), ecss)
+            def m(match_type='*', query='', action_data=''):
+                action = 'change' if action_data else 'remove'
+                self.ae(apply_rule(
+                    css, property=prop, match_type=match_type, query=query, action=action, action_data=action_data
+                ), ecss)
+
+            prop = 'font-size'
+            css, ecss = 'font-size: 1.2rem', 'font-size: 1.2em'
+            m('matches', query='(.+)rem', action_data=r'\1em')
 
             prop = 'color'
             css, ecss = 'color: red; margin: 0', 'margin: 0'
@@ -446,6 +457,7 @@ def test(return_tests=False):  # {{{
             m('line-height: 2', 'line-height: 1', '/', '2')
             prop = 'border-top-width'
             m('border-width: 1', 'border-bottom-width: 1;\nborder-left-width: 1;\nborder-right-width: 1;\nborder-top-width: 3', '*', '3')
+            prop = 'font-size'
 
         def test_export_import(self):
             rule = {'property':'a', 'match_type':'*', 'query':'some text', 'action':'remove', 'action_data':'color: red; a: b'}