Merge pull request #14 from a8m/feat/escape

parse: add support for escaping expressions

README.md

@@ -56,14 +56,16 @@ func main() {
 
 |__Expression__     | __Meaning__    |
 | ----------------- | -------------- |
-|`${var}`	   | Value of var (same as `$var`)
+|`${var}`           | Value of var (same as `$var`)
 |`${var-$DEFAULT}`  | If var not set, evaluate expression as $DEFAULT
 |`${var:-$DEFAULT}` | If var not set or is empty, evaluate expression as $DEFAULT
 |`${var=$DEFAULT}`  | If var not set, evaluate expression as $DEFAULT
 |`${var:=$DEFAULT}` | If var not set or is empty, evaluate expression as $DEFAULT
-|`${var+$OTHER}`	   | If var set, evaluate expression as $OTHER, otherwise as empty string
+|`${var+$OTHER}`    | If var set, evaluate expression as $OTHER, otherwise as empty string
 |`${var:+$OTHER}`   | If var set, evaluate expression as $OTHER, otherwise as empty string
-<sub>table taken from [here](http://www.tldp.org/LDP/abs/html/refcards.html#AEN22728)</sub>
+|`$$var`            | Escape expressions. Result will be `$var`. 
+
+<sub>Most of the rows in this table were taken from [here](http://www.tldp.org/LDP/abs/html/refcards.html#AEN22728)</sub>
 
 ### See also
 

parse/lex.go

@@ -144,17 +144,24 @@ Loop:
 		switch r := l.next(); r {
 		case '$':
 			l.pos--
+			// emit the text we've found until here, if any.
 			if l.pos > l.start {
 				l.emit(itemText)
 			}
 			l.pos++
-			if r := l.next(); isAlphaNumeric(r) {
-				l.backup()
-				return lexVariable
-			} else if r == '{' {
+			switch r := l.peek(); {
+			case r == '$':
+				// ignore the previous '$'.
+				l.ignore()
+				l.next()
+				l.emit(itemText)
+			case r == '{':
+				l.next()
 				l.subsDepth++
 				l.emit(itemLeftDelim)
 				return lexSubstitution
+			case isAlphaNumeric(r):
+				return lexVariable
 			}
 		case eof:
 			break Loop

parse/lex_test.go

@@ -81,13 +81,25 @@ var lexTests = []lexTest{
 		{itemVariable, 0, "world"},
 		{itemError, 0, "closing brace expected"},
 	}},
+	{"escaping $$var", "hello $$HOME", []item{
+		{itemText, 0, "hello "},
+		{itemText, 7, "$"},
+		{itemText, 8, "HOME"},
+		tEOF,
+	}},
+	{"escaping $${subst}", "hello $${HOME}", []item{
+		{itemText, 0, "hello "},
+		{itemText, 7, "$"},
+		{itemText, 8, "{HOME}"},
+		tEOF,
+	}},
 }
 
 func TestLex(t *testing.T) {
 	for _, test := range lexTests {
 		items := collect(&test)
 		if !equal(items, test.items, false) {
-			t.Errorf("%s: got\n\t%+v\nexpected\n\t%v", test.name, items, test.items)
+			t.Errorf("%s:\ninput\n\t%q\ngot\n\t%+v\nexpected\n\t%v", test.name, test.input, items, test.items)
 		}
 	}
 }

parse/parse_test.go

@@ -99,6 +99,12 @@ var parseTests = []parseTest{
 	{"$var and $DEFAULT empty :=", "${EMPTY:=$ALSO_EMPTY}", "", errEmpty},
 	{"$var and $OTHER empty +", "${EMPTY+$ALSO_EMPTY}", "", errEmpty},
 	{"$var and $OTHER empty :+", "${EMPTY:+$ALSO_EMPTY}", "", errEmpty},
+
+	// escaping.
+	{"escape $$var", "FOO $$BAR BAZ", "FOO $BAR BAZ", errNone},
+	{"escape $${subst}", "FOO $${BAR} BAZ", "FOO ${BAR} BAZ", errNone},
+	{"escape $$$var", "$$$BAR", "$bar", errNone},
+	{"escape $$${subst}", "$$${BAZ:-baz}", "$baz", errNone},
 }
 
 func TestParse(t *testing.T) {