[英] 用 regex 来查找文段中的文字效率很低，使用 FlashText 可提升 28x 速度

When developers work with text, they often need to clean it up first. Sometimes it’s by replacing keywords. Like replacing “Javascript” with “JavaScript”. Other times, we just want to find out whether “JavaScript” was mentioned in a document. Data cleaning tasks like these are standard for most Data Science projects dealing with text.

Data Science starts with data cleaning.

I had a very similar task to work on recently. I work as a Data Scientist at Belong.co and Natural Language Processing is half of my work.

When I trained a Word2Vec model on our document corpus, it started giving synonyms as similar terms. “Javascripting” was coming as a similar term to “JavaScript”.

To resolve this, I wrote a regular expression (Regex) to replace all known synonyms with standardized names. The Regex replaced “Javascripting” with “JavaScript”, which solved 1 problem but created another.

Some people, when confronted with a problem, think 
“I know, I’ll use regular expressions.” Now they have two problems.

The above quote is from this stack-exchange question and it came true for me. It turns out that Regex is fast if the number of keywords to be searched and replaced is in the 100s. But my corpus had over 10s of Thousands of keywords and a few Million documents.

When I benchmarked my Regex code, I found it was going to take 5 days to complete one run. My reaction:

Clearly something needed to be done.

I asked around in my office and on stack-overflow. And a couple of suggestions came up. Both Vinay, Suresh and Stack Overflow pointed towards this beautiful algorithm called Aho-Corasick algorithm and Trie dictionary approach. I looked for some existing solutions but couldn’t find much.

So I wrote my own implementation and FlashText was born.

Before we get into what is FlashText and how it works, let’s have a look at how it performs.

Time taken by FlashText to find terms in comparison to Regex.

The chart shown above is a comparison of Complied Regex against FlashText for 1 document. As the number of keywords increase, the time taken by Regex grows almost linearly. Yet with FlashText it doesn’t matter.

FlashText reduced our run time from 5 days to 15 minutes!!

Time taken by FlashText to replace terms in comparison to Regex.

Code used for the benchmark shown above is linked here, and here.

So what is FlashText?

FlashText is a Python library that I open sourced on GitHub. It is efficient at both extracting keywords and replacing them.

To use FlashText first you have to pass it a list of keywords. This list will be used internally to build a Trie dictionary. Then you pass a string to it and tell if you want to perform replace or search.

For replace it will create a new string with replaced keywords. For search it will return a list of keywords found in the string. This will all happen in one pass over the input string.

Here is what one happy user had to say about the library:

Radim Řehůřek @radimrehurek @arnicas 28x faster than a compiled regexp for 1k keywords & ~10k tokens per document. Nice @vi3k6i5!Pure Python t… twitter.com/i/web/statu… 08:48 AM - 05 Sep 2017 2 10

Radim Rehurek is the creator of Gensim.

Why is FlashText so fast ?

Let’s try and understand this part with an example. Say we have a sentence which has 3 words I like Python, and a corpus which has 4 words {Python, Java, J2ee, Ruby}.

If we take each word from the corpus, and check if it is present in sentence, it will take 4 tries.

is 'Python' in sentence? 
is 'Java' in sentence?
...

If the corpus had n words it would have taken n loops. Also each search step is <word> in sentence? will take its own time. This is kind of what happens in Regex match.

There is another approach which is reverse of the first one. For each word in the sentence, check if it is present in corpus.

is 'I' in corpus?
is 'like' in corpus?
is 'python' in corpus?

If the sentence had m words it would have taken m loops. In this case the time it takes is only dependent on the number of words in sentence. And this step, is <word> in corpus? can be made fast using a dictionary lookup.

FlashText algorithm is based on the second approach. It is inspired by the Aho-Corasick algorithm and Trie data structure.

The way it works is: First a Trie dictionary is created with the corpus. It will look somewhat like this

Start and EOT (End Of Term) represent word boundaries like space, period and new_line. A keyword will only match if it has word boundaries on both sides of it. This will prevent matching apple in pineapple.

Next we will take an input string I like Python and search it character by character.

Step 1: is <start>I<EOT> in dictionary? No
Step 2: is <start>like<EOT> in dictionary? No
Step 3: is <start>Python<EOT> in dictionary? Yes

Since this is a character by character match, we could easily skip <start>like<EOT> at <start>l because l is not connected to start. This makes skipping missing words really fast.

The FlashText algorithm only went over each character of the input string ‘I like Python’. The dictionary could have very well had a million keywords, with no impact on the runtime. This is the true power of FlashText algorithm.

So when should you use FlashText?

Simple Answer: When Number of keywords > 500

Complicated Answer: Regex can search for terms based on regular expression special characters like ^,$,*,\d,. all this is not supported in FlashText. All FlashText understands the Start and End of terms. Simply speaking it understands \w,\b. So it’s no good if you want to match partial words like word\dvec. But it is excellent for extracting complete words like word2vec.

How to use FlashText

To Find Terms:

	# pip install flashtext
	from flashtext.keyword import KeywordProcessor
	keyword_processor = KeywordProcessor()
	keyword_processor.add_keyword('Big Apple', 'New York')
	keyword_processor.add_keyword('Bay Area')
	keywords_found = keyword_processor.extract_keywords('I love Big Apple and Bay Area.')
	keywords_found
	# ['New York', 'Bay Area']

view raw flashtext_extract_example.pyhosted with ❤ by GitHub

To Replace Terms:

	from flashtext.keyword import KeywordProcessor
	keyword_processor = KeywordProcessor()
	keyword_processor.add_keyword('Big Apple', 'New York')
	keyword_processor.add_keyword('New Delhi', 'NCR region')
	new_sentence = keyword_processor.replace_keywords('I love Big Apple and new delhi.')
	new_sentence
	# 'I love New York and NCR region.'

view raw flashtext_replace_example.pyhosted with ❤ by GitHub

What Next?

If you know someone who works on Entity recognition, NLP, Word2vec, Keras, Tensorflow please share this blog with them. This library has been really useful for us. I am sure it would be useful for others also.

Originally posted here

:wq