Issue 569 - Support empty strings in filter query

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 ### Fixed
+[#569](https://github.com/plotly/dash-table/issues/569), [#544](https://github.com/plotly/dash-table/issues/544)
+- Allow empty strings in all `filter_query` (e.g filter_query: '{colA} eq ""')
+
 [#567](https://github.com/plotly/dash-table/issues/567)
 - Add support for missing `border-radius` in style_** props
 - Fix table's inner vs. outer container styling 

src/dash-table/syntax-tree/lexeme/expression.ts

@@ -4,7 +4,7 @@ import { LexemeType, IUnboundedLexeme } from 'core/syntax-tree/lexicon';
 import { ISyntaxTree } from 'core/syntax-tree/syntaxer';
 
 const FIELD_REGEX = /^{(([^{}\\]|\\.)+)}/;
-const STRING_REGEX = /^(('([^'\\]|\\.)+')|("([^"\\]|\\.)+")|(`([^`\\]|\\.)+`))/;
+const STRING_REGEX = /^(('([^'\\]|\\.)*')|("([^"\\]|\\.)*")|(`([^`\\]|\\.)*`))/;
 const VALUE_REGEX = /^(([^\s'"`{}()\\]|\\.)+)(?:[\s)]|$)/;
 
 const getField = (

tests/cypress/tests/unit/lexeme_test.ts

@@ -39,6 +39,7 @@ describe('expression', () => {
         expect(typeof stringExpression.resolve).to.equal('function');
 
         if (stringExpression.resolve) {
+            expect(stringExpression.resolve(undefined, { value: '\'\'' } as ISyntaxTree)).to.equal('');
             expect(stringExpression.resolve(undefined, { value: '\'abc\'' } as ISyntaxTree)).to.equal('abc');
             expect(stringExpression.resolve(undefined, { value: '"abc"' } as ISyntaxTree)).to.equal('abc');
             expect(stringExpression.resolve(undefined, { value: '`abc`' } as ISyntaxTree)).to.equal('abc');

tests/cypress/tests/unit/query_syntactic_tree_test.ts

@@ -264,6 +264,15 @@ describe('Query Syntax Tree', () => {
             expect(tree.evaluate(data2)).to.equal(false);
             expect(tree.evaluate(data3)).to.equal(false);
         });
+
+        it('can compare to empty string', () => {
+            const tree = new QuerySyntaxTree('{c} eq ""');
+
+            expect(tree.isValid).to.equal(true);
+            expect(tree.evaluate({ c: 'a' })).to.equal(false);
+            expect(tree.evaluate({ c: ' ' })).to.equal(false);
+            expect(tree.evaluate({ c: '' })).to.equal(true);
+        });
     });
 
     describe('block', () => {