Anchor the field name regular expression

README.md

@@ -71,7 +71,7 @@ Inside a paragraph, list item, or header, the following inline formatting elemen
 
 **Variables** are written as `_x_` and are translated to `<var>x</var>`. Variables cannot contain whitespace or other formatting characters.
 
-**Fields** are written as `[[f]]` and are translated as `<var class="field">[[f]]</var>`. Field names must match `/[a-zA-Z0-9_]+/`.
+**Fields** are written as `[[f]]` and are translated as `<var class="field">[[f]]</var>`. Field names must match regular expression `/^[a-zA-Z0-9_]+$/`.
 
 **Values** are written as `*x*` and are translated to `<emu-val>x</emu-val>`. Values cannot contain asterisks.