A regex literal that starts a statement is parsed as division, and the formatter removes the parentheses that prevent it

[nkrkv]

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• Is it a bug? Usage questions should often be asked in the forum instead.
• Concise, focused, friendly issue title & description.
• A minimal, reproducible example.
• OS and browser versions, if relevant.
• Is it already fixed in master? Instructions

A regex literal at the start of a statement, on the line after another statement, is parsed as a division that continues the previous line, so the file does not parse.
rescript format produces exactly this form from code that parses: it removes the parentheses around the literal.

Snippet

src/A.res:

let f = s => {
  let t = s
  (/a/)->RegExp.test(t)
}

npx rescript format src/A.res
cat src/A.res
npx rescript build

Actual

let f = s => {
  let t = s
  /a/->RegExp.test(t)
}
Cleaned 0/0
Error in check:

  Syntax error!
  /path/to/check/src/A.res:3:6-7

  1 │ let f = s => {
  2 │   let t = s
  3 │   /a/->RegExp.test(t)
  4 │ }
  5 │ 

  Did you forget to write an expression here?


Incremental build failed. Error:   Could not parse Source Files

The formatted form fails the same way when written by hand, and so does a regex literal after an expression statement (Console.log(s)) or after a top-level let.

The formatter gives the same result for the other spellings that parse:

• ; at the end of the line before the literal (let t = s;) is removed;
• on 12.3.1, %re("/a/")->RegExp.test(t) becomes /a/->RegExp.test(t).

These spellings parse and keep their form after formatting:

• the literal as the first statement of the block;
• the literal as a call argument: RegExp.test(/a/, t);
• the literal bound to a name first: let r = /a/.

Expected

The file parses: / at the start of a statement starts a regex literal.
If that is not possible, the formatter keeps the parentheses (or the ;) in front of such a literal.

Possible cause: in parse_binary_expr (compiler/syntax/src/res_core.ml), Minus | MinusDot | LessThan | Percent on a new line with no whitespace after it is not taken as a binary operator.
Forwardslash has no such case, so / on a new line always continues the expression on the line before.

Worked in ReScript 11

ReScript 11.1.4 has no regex literals, and its formatter keeps %re:

let f = s => {
  let t = s
  %re("/a/")->Js.Re.test_(t)
}

The same commands print the file unchanged and build it:

let f = s => {
  let t = s
  %re("/a/")->Js.Re.test_(t)
}
>>>> Start compiling
Dependency Finished
rescript: [1/3] src/A.ast
rescript: [2/3] src/A.d
rescript: [3/3] src/A.cmj
>>>> Finish compiling 19 mseconds

Environment

• ReScript 12.3.1, 13.0.0-alpha.6, and master 58c6c89 (from pkg.pr.new, reports 13.0.0-alpha.7): the same result on all three.
• Node.js 24.9.0
• Linux x86_64

Context

We hit it when rescript format 12.3.1 rewrote %re("/^[0-9\.,+-]*$/")->RegExp.test(str) ? Some(str) : None, which came right after a let line in a block.

Workaround

Pass the literal as an argument (RegExp.test(/a/, t)) or bind it with let first.