Regex engine

A small regex engine written in Python, follows standard regex syntax. The engine can be used to match a regex pattern to the beginning or end of a string or anywhere inside a given string.

The basic regex match function is:

regex(pattern, text, mode="standard")

The possible modes are standard which looks for an exact match between the pattern and the string, returning a Boolean, begin which matches the regex with the beginning of the string and returns the list of matching strings, end which matches with the end of a string and returns the list of matches, any which will look for matches between the pattern and every substring, returning a list of matches.

Example:

regex("abc*\d", "abccc1)

returns true

This function can be applied to more complex regex expressions, such as:

regex("^(a|b|c)(1|2|3)", "a23")

which returns true.

The function:

match_capture(pattern, text)

attempts to match the pattern with a user provided string passed as the second argument. It returns a list of captured strings, it will fail with an error message if a suitable match is not found for one of the expressions to capture. Backtracking is not performed and the operators are greedy by default.

As an example the calls:

match_capture("{A}{[0-9]*}-{[0-9]*}-{[0-9]*}", "A328-32-67"))

match_capture("{[a-z]*}-{[a-z]*}", "first-second")

will return:

['A', '328', '32', '67']

['first', 'second]

Features

Operators and quantifiers

All quantifiers are greedy in both the stardard regex mode and the match_capture function, backtracking is not supported.

• * -> match zero or more times
• + -> match one or more times
• ? -> match zero or one time
• | -> OR operator
• Ranges (syntax: /(expr)[n:m]/ ) will match the expression between n and m times

Groups

These constructs help with matching characters which belong to a common type.

• [0-9] -> match any digit
• [a-z] -> match any lower case character
• [A-Z] -> match any upper case character
• [a-zA-Z0-9] -> match any alphanumeric character
• [chars] matches any character in group enclosed by parenthesis, ex. [abcd]
• "." (wildcard) matches any character

Escapes

• \w -> matches any alphabetic character
• \d -> matches numeral character
• \s -> matches whitespace
• (reserved_character) -> will treat the reserved character as a literal to match

Modifiers

The modifiers can effectively call the regex() function with the "begin" or "end" mode without having to specify it in the function call.

• ^ causes the regex function to match the pattern to the beginning of a string (ex: "^ab")
• $ causes the regex function to match to the end of the string (ex: "abc$")