markdown-it/lib/helpers/parse_link_label.js

// Parse link label
//
// this function assumes that first character ("[") already matches;
// returns the end of the label
//
'use strict';

module.exports = function parseLinkLabel(state, start, disableNested) {
  var level, found, marker, prevPos,
      labelEnd = -1,
      max = state.posMax,
      oldPos = state.pos;

  state.pos = start + 1;
  level = 1;

  while (state.pos < max) {
    marker = state.src.charCodeAt(state.pos);
    if (marker === 0x5D /* ] */) {
      level--;
      if (level === 0) {
        found = true;
        break;
      }
    }

    prevPos = state.pos;
    state.md.inline.skipToken(state);
    if (marker === 0x5B /* [ */) {
      if (prevPos === state.pos - 1) {
        // increase level if we find text `[`, which is not a part of any token
        level++;
      } else if (disableNested) {
        state.pos = oldPos;
        return -1;
      }
    }
  }

  if (found) {
    labelEnd = state.pos;
  }

  // restore old state
  state.pos = oldPos;

  return labelEnd;
};