[GitHub] [arrow-datafusion] crepererum commented on a diff in pull request #6369: feat: add pattern for simplifying exprs like `str ~ '^foo$'`

["crepererum (via GitHub)" <gi...@apache.org>]

crepererum commented on code in PR #6369:
URL: https://github.com/apache/arrow-datafusion/pull/6369#discussion_r1196117383


##########
datafusion/optimizer/src/simplify_expressions/regex.rs:
##########
@@ -130,6 +139,46 @@ fn is_safe_for_like(c: char) -> bool {
     (c != '%') && (c != '_')
 }
 
+/// returns true if the elements in a `Concat` pattern are:
+/// - `[Look::Start, Look::End]`
+/// - `[Look::Start, Literal(_), Look::End]`
+fn is_anchored_literal(v: &[Hir]) -> bool {
+    match v.len() {
+        2..=3 => (),
+        _ => return false,
+    };
+
+    let first_last = (
+        v.first().expect("length checked"),
+        v.last().expect("length checked"),
+    );
+    if !matches!(first_last,
+    (s, e) if s.kind() == &HirKind::Look(Look::Start)
+        && e.kind() == &HirKind::Look(Look::End)
+         )
+    {
+        return false;
+    }
+
+    v.iter()
+        .skip(1)
+        .take(v.len() - 2)
+        .all(|h| matches!(h.kind(), HirKind::Literal(_)))
+}
+
+/// extracts a string literal expression assuming that `is_anchored_literal()`

Review Comment:
   ```suggestion
   /// extracts a string literal expression assuming that [`is_anchored_literal`]
   ```



##########
datafusion/optimizer/src/simplify_expressions/expr_simplifier.rs:
##########
@@ -2434,6 +2434,21 @@ mod tests {
         // single word
         assert_change(regex_match(col("c1"), lit("foo")), like(col("c1"), "%foo%"));
 
+        // regular expressions that match an exact literal
+        assert_change(regex_match(col("c1"), lit("^$")), col("c1").eq(lit("")));
+        assert_change(
+            regex_not_match(col("c1"), lit("^$")),
+            col("c1").not_eq(lit("")),
+        );
+        assert_change(
+            regex_match(col("c1"), lit("^foo$")),
+            col("c1").eq(lit("foo")),
+        );
+        assert_change(
+            regex_not_match(col("c1"), lit("^foo$")),
+            col("c1").not_eq(lit("foo")),
+        );

Review Comment:
   Could we have another "unsupported" test here for (use `assert_no_change`, see other cases in this test):
   
   - `^foo|bar$` (OR within anchor)
   - `^(foo)(bar)$` (I think this is a concat or more than 3 elements)
   - `^` and `$` (single element)
   - `$^` and `$foo^` (anchors flipped)



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