Problem Statement/Rationale

When exporting aggregation pipeline to some languages, Regex expression is escaped incorrectly

Steps to Reproduce

1. Create aggregation pipeline with a single $addFields stage:

   {
     matches: {$regexFindAll: {regex: /\d+/, input: "$data"}},
     justString: "\"",
   } 

1. Export it to Node.js, C#, Go, Rust, PHP, or Python.
2. Observe generated regex string.

Expected Results

Regex expression in these languages are generated as regular string, so to produce string "\d", we need to escape \, otherwise we will get just "d".
Nodejs: new RegExp('
d+')
C#: new Regex("
d+")Go: primitive.Regex{Pattern: "
d+"}
Rust: pattern: "
d+".to_string() *
PHP: new Regex("
d+")

* - note: in Rust there is r".." for regex strings, so maybe we can use pattern: r"\d+".to_string() instead.

Actual Results

We don't escape \, so instead of digits, we match just the letter d.
Nodejs: new RegExp('\d+')
C#: new Regex("\d+")Go: primitive.Regex{Pattern: "\d+"}
Rust: pattern: "\d+".to_string()
PHP: new Regex("\d+")