How does BERT word embedding preprocess work

BERT provides its own tokenizer. Because BERT is a pretrained model that expects input data in a specific format, following are required:

• A special token, [SEP], to mark the end of a sentence, or the separation between two sentences
• A special token, [CLS], at the beginning of our text. This token is used for classification tasks, but BERT expects it no matter what your application is.
• Tokens that conform with the fixed vocabulary used in BERT
• The Token IDs for the tokens, from BERT’s tokenizer
• Mask IDs to indicate which elements in the sequence are tokens and which are padding elements
• Segment IDs used to distinguish different sentences
• Positional Embeddings used to show token position within the sequence

.

from transformers import BertTokenizer

# Load pre-trained model tokenizer (vocabulary)
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')

# An example sentence 
text = "Sentence to embed"

# Add the special tokens.
marked_text = "[CLS] " + text + " [SEP]"

# Split the sentence into tokens.
tokenized_text = tokenizer.tokenize(marked_text)

# Map the token strings to their vocabulary indices.
indexed_tokens = tokenizer.convert_tokens_to_ids(tokenized_text) 

Have a look at this excellent tutorial for more details.