maybe we can add support for that in polars-bio if there is broader need for that ?

this does not necessarily answer your question, but becoming familiar with parsing CIGAR is sometimes quite useful

Here is a short code that can help (js/typescript)

https://cmdcolin.github.io/posts/2022-02-06-sv-sam#appendix-b---cigar-parsing

// @param cigar: CIGAR string in text form
// @returns an array of elements like ['30','M', '2','I', '50','M', '40','D'] which you can consume in a loop two elements at a time
function parseCigar(cigar: string) {
  return cigar.split(/([MIDNSHPX=])/)
}
// this function does nothing, but is informative for how to parse interpret a
// CIGAR string
// @param cigar:CIGAR string from record
// @param readSeq: the SEQ from record
// @param refSeq: the reference sequence underlying the read
function interpretCigar(cigar: string, readSeq: string, refSeq: string) {
  const opts = parseCigar(cigar)
  let qpos = 0 // query position, position on the read
  let tpos = 0 // target position, position on the reference sequence
  for (let i = 0; i < ops.length; i += 2) {
    const length = +opts[i]
    const operator = opts[i + 1]
    // do things. refer to the CIGAR chart in SAMv1.pdf for which operators
    // "consume reference" to see whether to increment
    if (op === 'M' || op === '=') {
      // matches consume query and reference
      const refMatch = refSeq.slice(tpos, tpos + len)
      const readMatch = readSeq.slice(qpos, qpos + len)
      for (let i = 0; i < len; i++) {
        if (refMatch[i] !== readMatch[i]) {
          // SNP at this position
        }
      }
      qpos += len
      tpos += len
    }
    if (op === 'I') {
      // insertions only consume query
      // sequence of the insertion from the read is
      const insSeq = readSeq.slice(qpos, qpos + len)
      qpos += len
    }
    if (op === 'D') {
      // deletions only consume reference
      // sequence of the deletion from the reference is
      const delSeq = refSeq.slice(tpos, tpos + len)
      tpad += len
    }
    if (op === 'N') {
      // skips only consume reference
      // skips are similar to deletions but are related to spliced alignments
      tpad += len
    }
    if (op === 'X') {
      // mismatch using the extended CIGAR format
      // could lookup the mismatch letter in a string containing the reference
      const mismatch = refSeq.slice(tpos, tpos + len)
      qpos += len
      tpos += len
    }
    if (op === 'H') {
      // does not consume query or reference
      // hardclip is just an indicator
    }
    if (op === 'S') {
      // softclip consumes query
      // below gets the entire soft clipped portion
      const softClipStr = readSeq.slice(qpos, qpos + len)
      qpos += len
    }
  }
}