反向正则表达式机器实现

Question

I'm trying to match a string starting from the last character to fail as soon as possible. This way I can fail a match with a custom string cstr (see specification below) with least amount of operations (4th property).

From a theoritical perspective the regex can be represented as a finite state mashine and the arrows can be flipped, creating the reversed regex.

I'm looking for an implementation of this. A library/program which I can give the string and the pattern. cstr is implemented in python, so if possible a python module. (For the curious i-th character is not calculated until needed.) For anything other I need to do much more work because of cstr 's calculation is hard to port to another language.

The implementation doesn't have to cover all latex syntax. I'm looking for the basics. No lookaheads or fancy stuff. See specification below.

I may be lacking common knowledge. Please do comment obvious things, too.

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Specification

The custom string cstr has the following properties:

1. String can be calculated in finite time.
2. String has finite length
3. The last character is known
4. Every previous character requires a costly calculation
5. Until the string is calculated fully, length is unknown

When the string is calcualted fully, I want to match it with a simple regex which may contain these from the syntax. No look aheads or fancy stuff.

• alphanumeric characters
• uinicode characters
• . , * , + , ? , \w , \W , [] , | , escape char \ , range specifitation with {, }

PS: This is not a homework question. I'm trying to formulate my question as clear as possible.

Answer 1

OP here. Here are some thougts:

• Since I'm looking for an unoptimized regex mashine, I have to build it myself, which takes time.

• Alternatively we can define an upperbound for cstr length and create all strings that matches given regex with length < upperbound. Then we put all solutions to a tire data structure and match it. This depends on the use case and maybe a cache can be involved.

• What I'm going for is python module greenery

from greenery import parse
pattern = parse.Pattern(...)
pattern.reversed()
...

this sometimes provieds a good matching experience. Sometimes not but it is ok for me.