(?<=    # Positive look-behind
b      # Word boundary
|       # Or
_       # Underscore
)       # End group
d      # Digit
(?=     # Positive look-ahead
b|_)   # Word boundary or underscore

const manualBoundaries = /d/g
const matches = [];
while ((match = manualBoundaries.exec(str)) !== null) {
  const m = match[0]
  const i = match.index
  if ((i == 0 || str[i - 1].match(/(W|_)/)) &&
      (i + m.length == str.length || str[i + m.length].match(/(W|_)/)))
    matches.push(m)
}

const matches = [];
str.replace(manualBoundaries, (m, i) => {
    if ((i == 0 || str[i - 1].match(/(W|_)/)) &&
        (i + m.length == str.length || str[i + m.length].match(/(W|_)/)))
      matches.push(m);
});