Regex to match/replace leading tabs without lookbehind-CodePudding

I am trying to match each \t in the leading whitespace of a line so I can replace them with two spaces. This is trivial with an unbounded (i.e., variable-length) lookbehind.

text.replace(/(?<=^\s*)\t/gm, '  ')

Unfortunately, this code is running on iOS, and for some reason, Safari and iOS have yet to implement lookbehinds, let alone unbounded lookbehinds.

I know there are workarounds for lookbehinds, but I can't seem to get the ones I've looked at to work.

I would rather not capture any characters aside from each tab, but if there's no other way, I could capture characters around the tabs in capture groups and add $1, etc, to my replacement string.

Example test code

const text = `
\t\ta
  \t  b
 \t  \t c\td  \te
`

const expected = `
    a
      b
        c\td  \te
`

// throws error in iOS, which does not support lookbehinds
// const regex = /(?<=^\s*)\t/gm;
const regex = /to-do/gm;

const result = text.replace(regex, '  ')

console.log(`Text: ${text}`)
console.log(`Expected: ${expected}`)
console.log(`Result: ${result}`)
console.log(JSON.stringify([ expected, result ], null, 2))

if (result === expected) {
  console.info('Success!