Added the ability to escape the '$' character before '(' as "$$" which will allow using the "$()" and "$(())" expressions of bash, sh, and other shells. (#467)

The '$' character is now escaped as the "$$" sequence when a non-zero-length "$$" character sequence ends with the "$(" sequence or the '(' character.
This allows the use of SHELL $(...) or $((...)) expressions within bjam expressions such as "actions { ... }" or "[ SHELL ... ]".

---------

Co-authored-by: René Ferdinand Rivera Morell <grafikrobot@gmail.com>

src/engine/function.cpp

@@ -2363,6 +2363,20 @@ static int32_t try_parse_variable( char const * * s_, char const * * string,
     VAR_PARSE_GROUP * out )
 {
     char const * s = *s_;
+    // escaping '$' with "$$" sequence when it needed (sequence of multiple "$$" before "$(" or '(')
+    if ( s[ 0 ] == '$' && s[ 1 ] == '$' )
+    {
+        // count repeats of "$$" and checks that sequence used before '(' or "$(" (when escaping needed)
+        for(s += 2; s[ 0 ] == '$' && s[ 1 ] == '$'; s += 2);
+        if(s[ 0 ] == '(' || (s[ 0 ] == '$' && s[ 1 ] == '(')) {
+            // save escaped '$'s as half of found "$$" sequence
+            size_t repeats = (s - *s_) / 2;
+            var_parse_group_maybe_add_constant( out, *string, s - repeats );
+            *string = s;
+        }
+        *s_ = s;
+        return 1;
+    }
     if ( s[ 0 ] == '$' && s[ 1 ] == '(' )
     {
         var_parse_group_maybe_add_constant( out, *string, s );

test/escaping_dollar_before_round_bracket.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+
+# Copyright Ivan Kotov 2025.
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE.txt or copy at
+# https://www.bfgroup.xyz/b2/LICENSE.txt)
+
+# Regression test. When Jamfile contained "using whatever ; " and the 'whatever'
+# module declared a project, then all targets in Jamfile were considered to be
+# declared in the project associated with 'whatever', not with the Jamfile.
+
+import BoostBuild
+
+t = BoostBuild.Tester(use_test_config=False)
+
+t.write("jamroot.jam", """\
+
+rule test {
+    value = zero ;
+    echo $(value) ;
+    echo $$(value) ;
+    echo $$$(value) ;
+    echo $$$$(value) ;
+    echo $$$$ ;
+}
+
+test a.test ;
+
+""")
+
+t.run_build_system(stdout="""zero
+$(value)
+$zero
+$$(value)
+$$$$
+...found 1 target...
+""")
+
+t.cleanup()

test/test_all.py

@@ -293,6 +293,7 @@ tests = ["abs_workdir",
          "dll_path",
          "double_loading",
          "duplicate",
+         "escaping_dollar_before_round_bracket",
          "example_libraries",
          "example_make",
          "exit_status",