Use a constant for KEYWORD_PATTERN.

Author: gkellogg

lib/context.js

@@ -27,6 +27,7 @@ const {
 
 const INITIAL_CONTEXT_CACHE = new Map();
 const INITIAL_CONTEXT_CACHE_MAX_SIZE = 10000;
+const KEYWORD_PATTERN = /^@[a-zA-Z]+$/;
 
 const api = {};
 module.exports = api;
@@ -397,7 +398,7 @@ api.createTermDefinition = ({
       'Invalid JSON-LD syntax; keywords cannot be overridden.',
       'jsonld.SyntaxError',
       {code: 'keyword redefinition', context: localCtx, term});
-  } else if(term.match(/@[a-zA-Z]+$/)) {
+  } else if(term.match(KEYWORD_PATTERN)) {
     // FIXME: remove logging and use a handler
     console.warn('WARNING: terms beginning with "@" are reserved' +
       ' for future use and ignored', {term});
@@ -490,7 +491,7 @@ api.createTermDefinition = ({
         'jsonld.SyntaxError', {code: 'invalid IRI mapping', context: localCtx});
     }
 
-    if(reverse.match(/@[a-zA-Z]+$/)) {
+    if(reverse.match(KEYWORD_PATTERN)) {
       // FIXME: remove logging and use a handler
       console.warn('WARNING: values beginning with "@" are reserved' +
         ' for future use and ignored', {reverse});
@@ -511,7 +512,7 @@ api.createTermDefinition = ({
     if(id === null) {
       // reserve a null term, which may be protected
       mapping['@id'] = null;
-    } else if(!api.isKeyword(id) && id.match(/@[a-zA-Z]+$/)) {
+    } else if(!api.isKeyword(id) && id.match(KEYWORD_PATTERN)) {
       // FIXME: remove logging and use a handler
       console.warn('WARNING: values beginning with "@" are reserved' +
         ' for future use and ignored', {id});
@@ -877,7 +878,7 @@ function _expandIri(activeCtx, value, relativeTo, localCtx, defined, options) {
   }
 
   // ignore non-keyword things that look like a keyword
-  if(value.match(/^@[a-zA-Z]+$/)) {
+  if(value.match(KEYWORD_PATTERN)) {
     return null;
   }