如何在正则表达式中表达“重复此部分”？

Question

Suppose I want to match a string like this:

123(432)123(342)2348(34)

I can match digits like 123 with [\\d]* and (432) with \\([\\d]+\\) .

How can match the whole string by repeating either of the 2 patterns?

I tried [[\\d]* | \\([\\d]+\\)]+ [[\\d]* | \\([\\d]+\\)]+ , but this is incorrect.

I am using python re module.

Answer 1

I think you need this regex:

"^(\d+|\(\d+\))+$"

and to avoid catastrophic backtracking you need to change it to a regex like this:

"^(\d|\(\d+\))+$"

Answer 2

You can use a character class to match the whole of string :

[\d()]+

But if you want to match the separate parts in separate groups you can use re.findall with a spacial regex based on your need, for example :

>>> import re
>>> s="123(432)123(342)2348(34)"
>>> re.findall(r'\d+\(\d+\)',s)
['123(432)', '123(342)', '2348(34)']
>>> 

Or :

>>> re.findall(r'(\d+)\((\d+)\)',s)
[('123', '432'), ('123', '342'), ('2348', '34')]

Or you can just use \\d+ to get all the numbers :

>>> re.findall(r'\d+',s)
['123', '432', '123', '342', '2348', '34']

If you want to match the patter \\d+\\(\\d+\\) repeatedly you can use following regex :

(?:\d+\(\d+\))+

Answer 3

You can achieve it with this pattern:

^(?=.)\d*(?:\(\d+\)\d*)*$

demo

(?=.) ensures there is at least one character (if you want to allow empty strings, remove it).

\\d*(?:\\(\\d+\\)\\d*)* is an unrolled sub-pattern. Explanation: With a bactracking regex engine, when you have a sub-pattern like (A|B)* where A and B are mutually exclusive (or at least when the end of A or B doesn't match respectively the beginning of B or A), you can rewrite the sub-pattern like this: A*(BA*)* or B*(AB*)* . For your example, it replaces (?:\\d+|\\(\\d+\\))* This new form is more efficient: it reduces the steps needed to obtain a match, it avoids a great part of the eventual bactracking.

Note that you can improve it more, if you emulate an atomic group (?>....) with this trick (?=(....))\\1 that uses the fact that a lookahead is naturally atomic:

^(?=.)(?=(\d*(?:\(\d+\)\d*)*))\1$

demo (compare the number of steps needed with the previous version and check the debugger to see what happens)

Note: if you don't want two consecutive numbers enclosed in parenthesis, you only need to change the quantifier * with + inside the non-capturing group and to add (?:\\(\\d+\\))? at the end of the pattern, before the anchor $ :

^(?=.)\d*(?:\(\d+\)\d+)*(?:\(\d+\))?$

or

^(?=.)(?=(\d*(?:\(\d+\)\d+)*(?:\(\d+\))?))\1$