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Python Pattern Matching

Project description

How to install

pip install pypama


This package provides a pattern matching for list of objects. Just as Regex provides regular expression for strings, this package provides regular expression for other type of lists.

An example is worth a thousand words: assume you have a list

>>> example_list = ['a', 'a', 1, '', None, 'b', 'c',  'e']

For some reason, you know that there is an int and a None, and you want to extract that number and the 2 strings following the None

>>> from pypama import build_pattern, is_int, is_none
>>> g = build_pattern((~is_int).star(False), '(', is_int, ')', '.*', is_none, '(',ANY,ANY, ')')
>>> g.match(example_list).groups()
[[1], ['b', 'c']]
  • ~is_int will matching anything that’s not an integer
  • .star(False): equivalent to *? in regular expressions: repeat as many as necessary
  • parenthesis are for capturing groups
  • .* is short for (match anything, repeatedly)

Therefore the pattern above reads as follow: match anything that’s not an int, repeatedly, until you find an int that you capture in group 1. Then match anything until you find a None. That must be followed by two elements that you capture in group 2.

This is therefore very similar to the re package, but applied to a list and with matching that goes beyond strings.


build_pattern(*args, **functions) where args is a list of strings or Pattern objects, and functions maps function names to executable.

The args can be a string or a list of strings containing:
  • <string>: will match the string inside <>
  • callable unction: X will match if function(X) returns True
  • X?: will be ignored or match X
  • X*: will match if X matches repeatedly (greedy)
  • X*?: will match if X matches repeatedly (non greedy)
  • (X): will capture the X in a group
  • (P<name>X): will capture X in a group named “name”
  • X|Y: will match if X or Y matches
  • \n: will match the nth group previously matched
  • <r:X>: will match if the word matches regexp X
  • <re:X>: same as <r:X>
  • <c:function> or <call:function>: will match if function(X) matches. If function does not exist in the namespace of pypama, add the definition in the function call eg build_pattern('<c:foo>', foo=lambda x:x<2) will match [1]
  • . will match any item
  • $ will match if at the end of the list
  • X{n} will match X exactly n times. n can be a list of integer
  • X!: will match if the item doesn’t match X (X must match exactly 1 element)
Both are equally valid and evaluate to the same result;
  • build_pattern('<hello>','<world>','.*')
  • build_pattern('<hello><world>.*')

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