diff --git a/openformats/formats/yaml/utils.py b/openformats/formats/yaml/utils.py
index ebacae0c..9d9852e0 100644
--- a/openformats/formats/yaml/utils.py
+++ b/openformats/formats/yaml/utils.py
@@ -142,14 +142,22 @@ def _is_custom_tag(self, tag):
         Detect custom tags, like:
             `foo: !bar test`
             `foo: !xml "<bar>Bar</bar>"`
+
+        The name of the custom tag can have any of the following characters:
+        `a-z`, `A-Z`, `0-9`, `_`, `.`, `:`, `-`.
+        In any other case, we return `False`.
+
         Built-in types, indicated by a `!!` prefix, will not be matched. We
         can't preserve the information whether a built-in tag like `!!str` was
         used for a value since the PyYAML library will tag such entries with
         the built-in identifier. For example `tag:yaml.org,2002:str`, not
         `!!str`.
         """
-        return tag.startswith('!') and not tag.startswith('!!')
 
+        return re.match(ensure_unicode(r'^\![a-zA-Z0-9_:.\-]*$'),
+                        tag,
+                        re.IGNORECASE)
+ 
     def construct_mapping(self, node, deep=True):
         """
         Override `yaml.SafeLoader.construct_mapping` to return for each item
diff --git a/openformats/tests/formats/yaml/files/1_el.yml b/openformats/tests/formats/yaml/files/1_el.yml
index bfb5dc88..45e24e9b 100644
--- a/openformats/tests/formats/yaml/files/1_el.yml
+++ b/openformats/tests/formats/yaml/files/1_el.yml
@@ -78,8 +78,8 @@ number: !!int 123 # Should ignore
 bin: !!binary aGVsbG8= # Should ignore
 
 # Custom tags with numbers and symbols
-context_string: !he:fd94;fd/la "el:context string"
-verbim_context_string: !contex!t545qa "el:verbim context string"
+context_string: !cs:fd-94_fd.dot. "el:context string"
+verbim_context_string: !context:t5-46_qa "el:verbim context string"
 context_on_nested_map:
   first: !first_context:54KJFLA95KJ4 "el:context in nested map"
   second: !second_context:FDKJ40DK "el:context in nested map"
diff --git a/openformats/tests/formats/yaml/files/1_en.yml b/openformats/tests/formats/yaml/files/1_en.yml
index 18431611..b751cca8 100644
--- a/openformats/tests/formats/yaml/files/1_en.yml
+++ b/openformats/tests/formats/yaml/files/1_en.yml
@@ -81,8 +81,8 @@ number: !!int 123 # Should ignore
 bin: !!binary aGVsbG8= # Should ignore
 
 # Custom tags with numbers and symbols
-context_string: !he:fd94;fd/la "context string"
-verbim_context_string: !<!contex!t545qa> "verbim context string"
+context_string: !cs:fd-94_fd.dot. "context string"
+verbim_context_string: !<!context:t5-46_qa> "verbim context string"
 context_on_nested_map:
   first: !first_context:54KJFLA95KJ4 "context in nested map"
   second: !second_context:FDKJ40DK "context in nested map"
diff --git a/openformats/tests/formats/yaml/files/1_en_exported.yml b/openformats/tests/formats/yaml/files/1_en_exported.yml
index 10fa97ab..4e324640 100644
--- a/openformats/tests/formats/yaml/files/1_en_exported.yml
+++ b/openformats/tests/formats/yaml/files/1_en_exported.yml
@@ -78,8 +78,8 @@ number: !!int 123 # Should ignore
 bin: !!binary aGVsbG8= # Should ignore
 
 # Custom tags with numbers and symbols
-context_string: !he:fd94;fd/la "context string"
-verbim_context_string: !contex!t545qa "verbim context string"
+context_string: !cs:fd-94_fd.dot. "context string"
+verbim_context_string: !context:t5-46_qa "verbim context string"
 context_on_nested_map:
   first: !first_context:54KJFLA95KJ4 "context in nested map"
   second: !second_context:FDKJ40DK "context in nested map"
diff --git a/openformats/tests/formats/yaml/files/1_en_exported_without_template.yml b/openformats/tests/formats/yaml/files/1_en_exported_without_template.yml
index b9df1d14..2a21c55f 100644
--- a/openformats/tests/formats/yaml/files/1_en_exported_without_template.yml
+++ b/openformats/tests/formats/yaml/files/1_en_exported_without_template.yml
@@ -39,8 +39,8 @@ alias_key:
 foo: !test 'bar'
 bar: !xml "foo <xml>bar</xml>"
 hello: World
-context_string: !he:fd94;fd/la "context string"
-verbim_context_string: !contex%21t545qa "verbim context string"
+context_string: !cs:fd-94_fd.dot. "context string"
+verbim_context_string: !context:t5-46_qa "verbim context string"
 context_on_nested_map:
   first: "context in nested map"
   second: "context in nested map"
diff --git a/openformats/tests/formats/yaml/files/1_tpl.yml b/openformats/tests/formats/yaml/files/1_tpl.yml
index 3c6e96b5..0a5efd5d 100644
--- a/openformats/tests/formats/yaml/files/1_tpl.yml
+++ b/openformats/tests/formats/yaml/files/1_tpl.yml
@@ -72,8 +72,8 @@ number: !!int 123 # Should ignore
 bin: !!binary aGVsbG8= # Should ignore
 
 # Custom tags with numbers and symbols
-context_string: 629aded197db84c4d323cd3ed4cf0485_tr
-verbim_context_string: d37275706209cb19ee621ff3835522fe_tr
+context_string: 17d854ee46fe3d3bc91fadb4cf4df426_tr
+verbim_context_string: 0a3c1ae205b2c0d09ab69ab540e481f2_tr
 context_on_nested_map:
   first: 95175e30d6fbfe0f658e75919cd4982c_tr
   second: dce391c231a7ad7fef9e6cc0ddcbf549_tr
diff --git a/openformats/tests/formats/yamlinternationalization/files/1_el.yml b/openformats/tests/formats/yamlinternationalization/files/1_el.yml
index 73116b28..7f45b6d8 100644
--- a/openformats/tests/formats/yamlinternationalization/files/1_el.yml
+++ b/openformats/tests/formats/yamlinternationalization/files/1_el.yml
@@ -102,8 +102,8 @@ en:
     other: el:other
 
   # context
-  context_string: !he:fd94;fd/la "el:context string"
-  verbim_context_string: !contex!t545qa "el:verbim context string"
+  context_string: !cs:fd-94_fd.dot. "el:context string"
+  verbim_context_string: !context:t5-46_qa "el:verbim context string"
   context_on_nested_map:
     first: !first_context:54KJFLA95KJ4 "el:context in nested map"
     second: !second_context:FDKJ40DK "el:context in nested map"
diff --git a/openformats/tests/formats/yamlinternationalization/files/1_en.yml b/openformats/tests/formats/yamlinternationalization/files/1_en.yml
index 361c8663..deb2a70e 100644
--- a/openformats/tests/formats/yamlinternationalization/files/1_en.yml
+++ b/openformats/tests/formats/yamlinternationalization/files/1_en.yml
@@ -106,8 +106,8 @@ en:
     other: other
 
   # context
-  context_string: !he:fd94;fd/la "context string"
-  verbim_context_string: !<!contex!t545qa> "verbim context string"
+  context_string: !cs:fd-94_fd.dot. "context string"
+  verbim_context_string: !<!context:t5-46_qa> "verbim context string"
   context_on_nested_map:
     first: !first_context:54KJFLA95KJ4 "context in nested map"
     second: !second_context:FDKJ40DK "context in nested map"
diff --git a/openformats/tests/formats/yamlinternationalization/files/1_en_exported.yml b/openformats/tests/formats/yamlinternationalization/files/1_en_exported.yml
index 97d22440..d603d85e 100644
--- a/openformats/tests/formats/yamlinternationalization/files/1_en_exported.yml
+++ b/openformats/tests/formats/yamlinternationalization/files/1_en_exported.yml
@@ -102,8 +102,8 @@ en:
     other: other
 
   # context
-  context_string: !he:fd94;fd/la "context string"
-  verbim_context_string: !contex!t545qa "verbim context string"
+  context_string: !cs:fd-94_fd.dot. "context string"
+  verbim_context_string: !context:t5-46_qa "verbim context string"
   context_on_nested_map:
     first: !first_context:54KJFLA95KJ4 "context in nested map"
     second: !second_context:FDKJ40DK "context in nested map"
diff --git a/openformats/tests/formats/yamlinternationalization/files/1_en_exported_without_template.yml b/openformats/tests/formats/yamlinternationalization/files/1_en_exported_without_template.yml
index 6726558e..1595af46 100644
--- a/openformats/tests/formats/yamlinternationalization/files/1_en_exported_without_template.yml
+++ b/openformats/tests/formats/yamlinternationalization/files/1_en_exported_without_template.yml
@@ -59,8 +59,8 @@ en:
   anchor_mapping:
     one: one
     other: other
-  context_string: !he:fd94;fd/la "context string"
-  verbim_context_string: !contex%21t545qa "verbim context string"
+  context_string: !cs:fd-94_fd.dot. "context string"
+  verbim_context_string: !context:t5-46_qa "verbim context string"
   context_on_nested_map:
     first: "context in nested map"
     second: "context in nested map"
diff --git a/openformats/tests/formats/yamlinternationalization/files/1_tpl.yml b/openformats/tests/formats/yamlinternationalization/files/1_tpl.yml
index b9484b76..85943df5 100644
--- a/openformats/tests/formats/yamlinternationalization/files/1_tpl.yml
+++ b/openformats/tests/formats/yamlinternationalization/files/1_tpl.yml
@@ -92,8 +92,8 @@ en:
   798cf4a4e275a90e80b0aac837f06793_pl
 
   # context
-  context_string: 629aded197db84c4d323cd3ed4cf0485_tr
-  verbim_context_string: d37275706209cb19ee621ff3835522fe_tr
+  context_string: 17d854ee46fe3d3bc91fadb4cf4df426_tr
+  verbim_context_string: 0a3c1ae205b2c0d09ab69ab540e481f2_tr
   context_on_nested_map:
     first: 95175e30d6fbfe0f658e75919cd4982c_tr
     second: dce391c231a7ad7fef9e6cc0ddcbf549_tr