Convert images in rich text areas that are references to a Document, to base64 for exporting

ImageToBase64.cls

/**
 * This code is untested.
 * Assumes references to images in rich text fields is in the form of 'https://<url>/<documentId>' and refers to a document record
 */

public class ImageToBase64{
        public void convertToBase64(Id[] recordIds){
                Account[] accts = new Account[]{};
                Map<Id,Id[]> acctDocMap = new Map<Id,Id[]>();
                Map<Id,Document> docMap = new Map<Id,Document>();
                Id[] alldocIds = new Id[]{};
                for(Account a : [Select RichText__c from Account where Id in :recordIds]){
                        Pattern p = Pattern.compile('<img[^>]*?src="([^">]*)"[^>]*?>');
                        Matcher m = p.matcher(a.RichText__c);
                        Pattern urlSuffix = Pattern.compile('/([^/]*)$');
                        while(m.find()){
                                String imageUrl = m.group(1);
                                Matcher m2 = urlSuffix.matcher(imageUrl);
                                if(!m2.find() || String.isBlank(m2.group(1)))
                                        continue;
                                Id docId = m2.group(1);
                                alldocIds.add(docId);
                                if(!acctDocMap.containsKey(a.Id)
                                        acctDocMap.put(a.Id,new Id[]{docId});
                                else
                                        acctDocMap.get(a.Id).add(docId);
                        }
                }

                if(acctDocMap.size()>0){
                        docMap = new Map<Id,Document>([Select Type,Body from Document where Id in :alldocIds()]);
                        for(Id acctId: acctDocMap.keySet()){
                                Account a = acctDocMap.get(acctId);
                                m = p.matcher(a.RichText__c);
                                String newText='';
                                Integer lastMatchIndex=0;
                                while(m.find()){
                                        String imageUrl = m.group(1);
                                        Matcher m2 = urlSuffix.matcher(imageUrl);
                                        Id docId = m2.group(1);
                                        if(!docMap.containsKey(docId)){
                                                newText += a.RichText__c.substring(lastMatchIndex,m.end(1));
                                        }
                                        else{
                                                Document d = docMap.get(docId);
                                                Blob b = doc.Body;
                                                String base64Str = EncodingUtil.base64Encode(doc.Body);
                                                newText += a.RichText__c.substring(lastMatchIndex,m.start(1))+imageUrl.replaceFirst('data/'+doc.Type+';'+base64Str);
                                        }
                                        lastMatchIndex=m.end(1);
                                }
                                newText += a.RichText__c.substring(lastMatchIndex);
                                a.RichText__c = newText;
                                accts.add(a);
                        }
                }
                update accts;
        }
}

For exporting RTA with images and importing to another environment