syntax: add look-behinds cargo feature

Author: shilangyu

regex-syntax/Cargo.toml

@@ -35,6 +35,8 @@ unicode-perl = []
 unicode-script = []
 unicode-segment = []
 
+look-behinds = []
+
 [dependencies]
 arbitrary = { version = "1.3.0", features = ["derive"], optional = true }
 

regex-syntax/src/lib.rs

@@ -162,6 +162,8 @@ The following features are available:
   [`arbitrary`](https://crates.io/crates/arbitrary)
   crate. Namely, it implements the `Arbitrary` trait from that crate for the
   [`Ast`](crate::ast::Ast) type. This feature is disabled by default.
+* **look-behinds** -
+  Enable support for look-behind expressions. This feature is disabled by default.
 */
 
 #![no_std]

regex-syntax/test

@@ -20,6 +20,7 @@ features=(
     unicode-perl
     unicode-script
     unicode-segment
+    look-behinds
 )
 for f in "${features[@]}"; do
     echo "=== FEATURE: $f ==="