diff --git a/src/commands.rs b/src/commands.rs
index 314712b..680664e 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -73,6 +73,22 @@ fn take_until<'a>(sep: char, s: Input) -> Progress<String> {
     Ok((s, vec.into_iter().collect()))
 }
 
+// convert sed \1 syntax to regex crate $1 and escape $
+pub fn clean_replacement(mut s: String) -> String {
+    let mut dest = String::new();
+
+    // TODO static regexen
+    let dollar = Regex::new(r"\$").unwrap();
+    let changed = regex::replace_all(&dollar, &s, &mut dest, "$$$$");
+    if changed {std::mem::swap(&mut s, &mut dest)}
+
+    let backslash_digits = Regex::new(r"\\([0-9]+)").unwrap();
+    let changed = regex::replace_all(&backslash_digits, &s, &mut dest, r"$${$1}");
+    if changed {std::mem::swap(&mut s, &mut dest)}
+
+    s
+}
+
 pub fn parse_function<'a>(cmd: Input<'a>) -> Progress<Function> {
     let (s, function) = anychar(cmd)?;
     use Function::{*};
@@ -92,7 +108,7 @@ pub fn parse_function<'a>(cmd: Input<'a>) -> Progress<Function> {
             let regex = Regex::new(&format!("{}", ast)).unwrap();
             let (s, replacement) = take_until(sep, s)?;
             let (s, _) = char(sep)(s)?;
-            Ok((s, Fs(regex, replacement)))
+            Ok((s, Fs(regex, clean_replacement(replacement))))
         },
         'x' => Ok((s, Fx)),
         _ => fail(cmd)
@@ -216,4 +232,19 @@ pub mod tests {
     fn addr_comma() {
         address_equivalent("\\,foo,", &Context(dummy_regex()))
     }
+
+    #[test]
+    fn clean_noop() {
+        assert_eq!(clean_replacement("foo".to_string()), "foo")
+    }
+
+    #[test]
+    fn clean_ref() {
+        assert_eq!(clean_replacement(r"foo\1".to_string()), "foo${1}")
+    }
+
+    #[test]
+    fn clean_dollar() {
+        assert_eq!(clean_replacement("$foo".to_string()), "$$foo")
+    }
 }
diff --git a/src/regex/equivalent.rs b/src/regex/equivalent.rs
index eca4c00..33b3292 100644
--- a/src/regex/equivalent.rs
+++ b/src/regex/equivalent.rs
@@ -14,7 +14,7 @@ impl Equivalent for Ast {
             (Ast::Assertion(_), Ast::Assertion(_)) => panic!("not implemented"),
             (Ast::Class(_), Ast::Class(_)) => panic!("not implemented"),
             (Ast::Repetition(a), Ast::Repetition(b)) => a.equivalent(b),
-            (Ast::Group(_), Ast::Group(_)) => panic!("not implemented"),
+            (Ast::Group(a), Ast::Group(b)) => a.equivalent(b),
             (Ast::Alternation(_), Ast::Alternation(_)) => panic!("not implemented"),
             (Ast::Concat(a), Ast::Concat(b)) => a.equivalent(b),
             _ => false
@@ -40,6 +40,12 @@ impl Equivalent for RepetitionOp {
     }
 }
 
+impl Equivalent for Group {
+    fn equivalent(&self, other: &Group) -> bool {
+        self.kind == other.kind && self.ast.equivalent(&other.ast)
+    }
+}
+
 impl Equivalent for Concat {
     fn equivalent(&self, other: &Concat) -> bool {
         self.asts.len() == other.asts.len() && std::iter::zip(&self.asts, &other.asts).all(|(a, b)| a.equivalent(&b))
diff --git a/src/regex/parser.rs b/src/regex/parser.rs
index 5474021..1490c8c 100644
--- a/src/regex/parser.rs
+++ b/src/regex/parser.rs
@@ -1,5 +1,4 @@
 extern crate nom;
-
 use nom::character::complete::{char, none_of, one_of, u32};
 use nom::branch::alt;
 use nom::error::{ Error, ErrorKind};
@@ -9,7 +8,7 @@ use nom::{
     Err, Finish, IResult,
 };
 use nom_locate::{LocatedSpan};
-use regex_syntax::ast::{Ast, Concat, Literal, LiteralKind, Position, Repetition, RepetitionKind, RepetitionOp, RepetitionRange, Span};
+use regex_syntax::ast::{Ast, Concat, Group, GroupKind, Literal, LiteralKind, Position, Repetition, RepetitionKind, RepetitionOp, RepetitionRange, Span};
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct ExtraState {
@@ -85,9 +84,24 @@ fn escaped_literal(s: Input<'_>) -> Progress {
     })))
 }
 
+fn group(s: Input) -> Progress {
+    let start = position(s);
+    let (s, _) = char( '(' )(s)?;
+    // TODO named & non-capturing
+    let (s, ast) = branch(s)?;
+    let (mut s, _) = char( ')' )(s)?;
+    let end = position(s);
+    s.extra.last_regex += 1;
+    Ok((s, Ast::Group( Group {
+        span: Span{ start: start, end: end},
+        kind: GroupKind::CaptureIndex(s.extra.last_regex),
+        ast: Box::new(ast),
+    })))
+}
+
 fn atom(s: Input<'_>) -> Progress {
     // TODO () ^ $ \^.[$()|*+?{\ \
-    alt((literal, escaped_literal, dot))(s)
+    alt((group, literal, escaped_literal, dot))(s)
 }
 
 fn char_quantifier(s: Input<'_>) -> IResult<Input, RepetitionOp> {
@@ -240,5 +254,10 @@ pub mod tests {
     fn range() {
         match_modern_syntax("x{2,5}")
     }
+
+    #[test]
+    fn group() {
+        match_modern_syntax("(a*)")
+    }
     
 }