Replace template variables

Write a function `replace_template(template, variables)` that takes a string `template` and a dictionary `variables` mapping variable names to string values. The function must return a new string where every occurrence of `{{name}}` in the template is replaced by the corresponding value from `variables`. Variable names consist only of lowercase letters, digits, and underscores, and appear between double curly braces. If a variable name in the template is not present in `variables`, the placeholder `{{name}}` must be left unchanged in the output. All occurrences of a known variable must be replaced. The function should not modify the input dictionary or the template.

['>>> replace_template("Hello {{name}}!", {"name": "World"})\n"Hello World!"', '>>> replace_template("Hi {{name}} and {{name}}", {"name": "Alex"})\n"Hi Alex and Alex"', '>>> replace_template("{{greeting}}, {{name}}!", {"name": "Bob", "greeting": "Hello"})\n"Hello, Bob!"', '>>> replace_template("{{unknown}} value", {"known": "x"})\n"{{unknown}} value"', '>>> replace_template("No placeholders", {})\n"No placeholders"']