Return empty string from STRAFTER/STRBEFORE when there is no match

lib/query_processing/src/expressions/functions/str_after.rs

@@ -8,7 +8,8 @@ pub fn str_after(c: Column, s: String) -> Result<Column, PolarsError> {
             if let Some(range_to) = range_to {
                 Some(&x[range_to + s.len()..])
             } else {
-                Some(x)
+                // SPARQL 1.1 17.4.3.9/17.4.3.10: no occurrence gives the empty string
+                Some("")
             }
         } else {
             None

lib/query_processing/src/expressions/functions/str_before.rs

@@ -8,7 +8,8 @@ pub fn str_before(c: Column, s: String) -> Result<Column, PolarsError> {
             if let Some(range_to) = range_to {
                 Some(&x[0..range_to])
             } else {
-                Some(x)
+                // SPARQL 1.1 17.4.3.9/17.4.3.10: no occurrence gives the empty string
+                Some("")
             }
         } else {
             None

py_maplib/tests/test_str_before_after.py

@@ -0,0 +1,51 @@
+import pytest
+
+from maplib import Model
+
+# SPARQL 1.1 17.4.3.9 (STRBEFORE) and 17.4.3.10 (STRAFTER):
+# "If there is no such occurrence, an empty simple literal is returned."
+
+
+@pytest.mark.parametrize(
+    "expression,expected",
+    [
+        ('STRAFTER("abc", "b")', "c"),
+        ('STRAFTER("abc", "xyz")', ""),
+        ('STRAFTER("abc", "")', "abc"),
+        ('STRBEFORE("abc", "b")', "a"),
+        ('STRBEFORE("abc", "xyz")', ""),
+        ('STRBEFORE("abc", "")', ""),
+    ],
+)
+def test_str_before_after_simple_literal(expression, expected):
+    m = Model()
+    df = m.query(f"SELECT (STR({expression}) AS ?s) WHERE {{}}")
+    assert df["s"].to_list() == [expected]
+
+
+@pytest.mark.parametrize(
+    "expression,expected",
+    [
+        ('STRAFTER("abc"@en, "b")', "c"),
+        ('STRAFTER("abc"@en, "z")', ""),
+        ('STRBEFORE("abc"@en, "b")', "a"),
+        ('STRBEFORE("abc"@en, "z")', ""),
+    ],
+)
+def test_str_before_after_lang_string(expression, expected):
+    m = Model()
+    df = m.query(f"SELECT (STR({expression}) AS ?s) WHERE {{}}")
+    assert df["s"].to_list() == [expected]
+
+
+def test_str_after_no_match_on_data():
+    m = Model()
+    m.insert(
+        """PREFIX ex: <http://example.org/>
+        CONSTRUCT { ex:a ex:v "prefix_a" . ex:b ex:v "other_b" . } WHERE {}"""
+    )
+    df = m.query(
+        """PREFIX ex: <http://example.org/>
+        SELECT ?x (STRAFTER(?v, "prefix_") AS ?s) WHERE { ?x ex:v ?v } ORDER BY ?x"""
+    )
+    assert df["s"].to_list() == ["a", ""]