re: Add support for start- and endpos.

greezybacon · greezybacon · commit dbe3f83fc1f1 · 2025-07-18T21:07:32.000-05:00
Pattern objects have two additional parameters for the ::search and
::match methods to define the starting and ending position of the
subject within the string to be searched.

This allows for searching a sub-string without creating a slice.
However, one caveat of using the start-pos rather than a slice is that
the start anchor (`^`) remains anchored to the beginning of the text.

Signed-off-by: Jared Hancock &lt;jared@greezybacon.me&gt;
diff --git a/docs/library/re.rst b/docs/library/re.rst
@@ -154,15 +154,25 @@ Regex objects
 Compiled regular expression. Instances of this class are created using
 `re.compile()`.
 
-.. method:: regex.match(string)
-            regex.search(string)
+.. method:: regex.match(string, [pos, [endpos]])
+            regex.search(string, [pos, [endpos]])
             regex.sub(replace, string, count=0, flags=0, /)
 
    Similar to the module-level functions :meth:`match`, :meth:`search`
    and :meth:`sub`.
    Using methods is (much) more efficient if the same regex is applied to
    multiple strings.
 
+   The optional second parameter *pos* gives an index in the string where the
+   search is to start; it defaults to ``0``. This is not completely equivalent
+   to slicing the string; the ``'^'`` pattern character matches at the real
+   beginning of the string and at positions just after a newline, but not
+   necessarily at the index where the search is to start.
+
+   The optional parameter *endpos* limits how far the string will be searched;
+   it will be as if the string is *endpos* characters long, so only the
+   characters from *pos* to ``endpos - 1`` will be searched for a match.
+
 .. method:: regex.split(string, max_split=-1, /)
 
    Split a *string* using regex. If *max_split* is given, it specifies
diff --git a/extmod/modre.c b/extmod/modre.c
@@ -196,17 +196,42 @@ static void re_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t
 
 // Note: this function can't be named re_exec because it may clash with system headers, eg on FreeBSD
 static mp_obj_t re_exec_helper(bool is_anchored, uint n_args, const mp_obj_t *args) {
-    (void)n_args;
     mp_obj_re_t *self;
+    bool was_compiled = false;
     if (mp_obj_is_type(args[0], (mp_obj_type_t *)&re_type)) {
         self = MP_OBJ_TO_PTR(args[0]);
+        was_compiled = true;
     } else {
         self = MP_OBJ_TO_PTR(mod_re_compile(1, args));
     }
     Subject subj;
     size_t len;
     subj.begin_line = subj.begin = mp_obj_str_get_data(args[1], &len);
     subj.end = subj.begin + len;
+
+    if (was_compiled && n_args > 2) {
+        // Arg #2 is starting-pos
+        mp_int_t startpos = mp_obj_get_int(args[2]);
+        if (startpos > (mp_int_t) len) {
+            startpos = len;
+        }
+        else if (startpos < 0) {
+            startpos = 0;
+        }
+        subj.begin += startpos;
+        if (n_args > 3) {
+            // Arg #3 is ending-pos
+            mp_int_t endpos = mp_obj_get_int(args[3]);
+            if (endpos > (mp_int_t) len) {
+                endpos = len;
+            }
+            else if (endpos < startpos) {
+                endpos = startpos;
+            }
+            subj.end = subj.begin_line + endpos;
+        }
+    }
+
     int caps_num = (self->re.sub + 1) * 2;
     mp_obj_match_t *match = m_new_obj_var(mp_obj_match_t, caps, char *, caps_num);
     // cast is a workaround for a bug in msvc: it treats const char** as a const pointer instead of a pointer to pointer to const char
diff --git a/tests/extmod/re_start_end_pos.py b/tests/extmod/re_start_end_pos.py
@@ -0,0 +1,78 @@
+# test start and end pos specification
+
+try:
+    import re
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+def print_groups(match):
+    print("----")
+    try:
+        if match is not None:
+            i = 0
+            while True:
+                print(match.group(i))
+                i += 1
+    except IndexError:
+        pass
+
+
+p = re.compile(r"o")
+m = p.match("dog")
+print_groups(m)
+
+m = p.match("dog", 1)
+print_groups(m)
+
+m = p.match("dog", 2)
+print_groups(m)
+
+# No match past end of input
+m = p.match("dog", 5)
+print_groups(m)
+
+m = p.match("dog", 0, 1)
+print_groups(m)
+
+# Caret only matches the actual beginning
+p = re.compile(r"^o")
+m = p.match("dog", 1)
+print_groups(m)
+
+# End at beginning means searching empty string
+p = re.compile(r"o")
+m = p.match("dog", 1, 1)
+print_groups(m)
+
+# End before the beginning doesn't match anything
+m = p.match("dog", 2, 1)
+print_groups(m)
+
+# Negative starting values don't crash
+m = p.search("dog", -2)
+print_groups(m)
+
+m = p.search("dog", -2, -5)
+print_groups(m)
+
+# Search also works
+print("--search")
+
+p = re.compile(r"o")
+m = p.search("dog")
+print_groups(m)
+
+m = p.search("dog", 1)
+print_groups(m)
+
+m = p.search("dog", 2)
+print_groups(m)
+
+# Negative starting values don't crash
+m = p.search("dog", -2)
+print_groups(m)
+
+m = p.search("dog", -2, -5)
+print_groups(m)
