dfilter: Support more C escape sequences in string literals

Before:

  Filter: http.request.method == "\tHEAD"

  Constants:
  00000 PUT_FVALUE	"tHEAD" <FT_STRING> -> reg#1
  (...)

  Filter: http.request.method == "\uHEAD"

  Constants:
  00000 PUT_FVALUE	"uHEAD" <FT_STRING> -> reg#1
  (...)

After:

  Filter: http.request.method == "\tHEAD"

  Constants:
  00000 PUT_FVALUE	"\x09HEAD" <FT_STRING> -> reg#1
  (...)

  Filter: http.request.method == "\uHEAD"

  Constants:
  00000 PUT_FVALUE	"uHEAD" <FT_STRING> -> reg#1
  (...)

docbook/release-notes.adoc

@@ -116,6 +116,9 @@ The following features are new (or have been significantly updated) since versio
 ** Use the syntax "a ~= b" or "a any_ne b" to recover the previous (inconsistent with ==) logic for not equal.
 ** Adds support for the syntax "a not in b" as a synonym for "not a in b".
 ** Set elements must be separated using a comma, e.g: {1, 2, "foo"}. Using only whitespace as separator was deprecated in 3.6 and is now a syntax error.
+** Adds support for some additional character escape sequences in double quoted strings. Besides octal and hex byte specification the following C escape
+   sequences are now supported with the same meaning: \a, \b, \f, \n, \r, \t, \v. Previously they were only supported with character constants. Note that
+   unrecognized escape sequences are treated as a literal character. This has not changed from previous versions.
 
 * Corrected calculation of mean jitter in RTP Stream Analysis dialog and IAX2 Stram Analysis dialog
 

epan/dfilter/scanner.l

@@ -79,6 +79,7 @@ df_lval_t *df_lval;
 
 static int set_lval_str(int token, const char *token_value);
 #define simple(token) set_lval_str(token, yytext)
+static GString *append_escaped_char(GString *str, char c);
 
 /*
  * Sleazy hack to suppress compiler warnings in yy_fatal_error().
@@ -228,6 +229,10 @@ static int set_lval_str(int token, const char *token_value);
 
 <DQUOTE>\\x[[:xdigit:]]{1,2} {
 	/* hex sequence */
+	/*
+	 * C standard does not place a limit on the number of hex
+	 * digits after \x... but we do. \xNN can have 1 or two Ns, not more.
+	 */
 	if (yyextra->raw_string) {
 		g_string_append(yyextra->quoted_string, yytext);
 	}
@@ -251,7 +256,7 @@ static int set_lval_str(int token, const char *token_value);
 		g_string_append(yyextra->quoted_string, yytext);
 	}
 	else {
-		g_string_append_c(yyextra->quoted_string, yytext[1]);
+		append_escaped_char(yyextra->quoted_string, yytext[1]);
 	}
 }
 
@@ -342,3 +347,40 @@ set_lval_str(int token, const char *token_value)
 	df_lval->value = g_strdup(token_value);
 	return token;
 }
+
+static GString *
+append_escaped_char(GString *str, char c)
+{
+	switch (c) {
+		case 'a':
+			c = '\a';
+			break;
+		case 'b':
+			c = '\b';
+			break;
+		case 'f':
+			c = '\f';
+			break;
+		case 'n':
+			c = '\n';
+			break;
+		case 'r':
+			c = '\r';
+			break;
+		case 't':
+			c = '\t';
+			break;
+		case 'v':
+			c = '\v';
+			break;
+		default:
+			/* Unrecognized escapes are treated as a literal character.
+			 * If this is turned into an error instead (which is a backward
+			 * incompatibility but arguably the right thing to do)
+			 * we need to take care to accept all valid sequences
+			 * (like \" and \\). */
+			break;
+	}
+
+	return g_string_append_c(str, c);
+}