[PR] fix: align regexp_instr empty pattern [datafusion]

["Sean-Kenneth-Doherty (via GitHub)" <gi...@apache.org>]

Sean-Kenneth-Doherty opened a new pull request, #22286:
URL: https://github.com/apache/datafusion/pull/22286

   ## Which issue does this PR close?
   
   - Closes #22257.
   
   ## Rationale for this change
   
   PostgreSQL treats an empty regular expression in `regexp_instr` as a zero-width match. DataFusion previously special-cased empty patterns to return `0`, so `regexp_instr('abc', '')` diverged from PostgreSQL.
   
   ## What changes are included in this PR?
   
   - Handles empty `regexp_instr` patterns as zero-width matches at `start + N - 1`, returning `0` when that position is past the end of the string.
   - Keeps existing regex/flag validation for empty patterns.
   - Lets start/N validation run before empty-string handling.
   - Adds Rust unit coverage and sqllogictest coverage for empty-pattern behavior.
   
   ## Are these changes tested?
   
   - `cargo fmt --all`
   - `cargo test -p datafusion-functions regex::regexpinstr::tests::test_regexp_instr -- --nocapture`
   - `cargo test --profile=ci --test sqllogictests -- regexp/regexp_instr.slt`
   - `TMPDIR=/home/sean/Projects/datafusion-contrib/target/tmp cargo clippy --all-targets --all-features -- -D warnings`
   - `git diff --check`
   
   ## Are there any user-facing changes?
   
   Yes. `regexp_instr` now matches PostgreSQL behavior for empty regular expression patterns.
   


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Re: [PR] fix: align regexp_instr empty pattern [datafusion]

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

Jefffrey commented on code in PR #22286:
URL: https://github.com/apache/datafusion/pull/22286#discussion_r3339761870


##########
datafusion/functions/src/regex/regexpinstr.rs:
##########
@@ -412,8 +408,22 @@ where
         ));
     }
 
-    // --- Simplified byte_start_offset calculation ---

Review Comment:
   bit curious why we're removing these comments?



##########
datafusion/functions/src/regex/regexpinstr.rs:
##########
@@ -412,8 +408,22 @@ where
         ));
     }
 
-    // --- Simplified byte_start_offset calculation ---
     let total_chars = value.chars().count() as i64;
+    if pattern.is_empty() {
+        compile_and_cache_regex(pattern, flags, regex_cache)?;

Review Comment:
   is there a need for this compile+cache regex call here if the pattern is empty and not being used?



##########
datafusion/functions/src/regex/regexpinstr.rs:
##########
@@ -762,6 +773,22 @@ mod tests {
         });
     }
 
+    fn test_regexp_instr_empty_pattern_global_flag() {
+        let args = [
+            ScalarValue::Utf8(Some("abc".to_string())),
+            ScalarValue::Utf8(Some("".to_string())),
+            ScalarValue::Int64(Some(1)),
+            ScalarValue::Int64(Some(1)),
+            ScalarValue::Utf8(Some("g".to_string())),
+        ];
+        let err = regexp_instr_with_scalar_values(&args)
+            .expect_err("global flag should be rejected for empty patterns");

Review Comment:
   we reject global flag regardless of empty pattern, so not sure why this is being added like so



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