regex: remove error when accessing an unset group (JuliaLang#40391)

closes JuliaLang#31456

base/pcre.jl

@@ -228,7 +228,10 @@ function substring_length_bynumber(match_data, number)
     s = RefValue{Csize_t}()
     rc = ccall((:pcre2_substring_length_bynumber_8, PCRE_LIB), Cint,
                (Ptr{Cvoid}, Cint, Ref{Csize_t}), match_data, number, s)
-    rc < 0 && error("PCRE error: $(err_message(rc))")
+    if rc < 0
+        rc == ERROR_UNSET && return 0
+        error("PCRE error: $(err_message(rc))")
+    end
     return Int(s[])
 end
 

base/regex.jl

@@ -528,6 +528,8 @@ replace_err(repl) = error("Bad replacement string: $repl")
 
 function _write_capture(io, re::RegexAndMatchData, group)
     len = PCRE.substring_length_bynumber(re.match_data, group)
+    # in the case of an optional group that doesn't match, len == 0
+    len == 0 && return
     ensureroom(io, len+1)
     PCRE.substring_copy_bynumber(re.match_data, group,
         pointer(io.data, io.ptr), len+1)

test/strings/util.jl

@@ -275,6 +275,11 @@ end
     # Issue 13332
     @test replace("abc", 'b' => 2.1) == "a2.1c"
 
+    # Issue 31456
+    @test replace("The fox.", r"fox(es)?" => s"bus\1") == "The bus."
+    @test replace("The foxes.", r"fox(es)?" => s"bus\1") == "The buses."
+    @test replace("The quick fox quickly.", r"(quick)?\sfox(es)?\s(run)?" => s"\1 bus\2 \3") == "The quick bus quickly."
+
     # test replace with a count for String and GenericString
     # check that replace is a no-op if count==0
     for s in ["aaa", Test.GenericString("aaa")]