python中的正则表达式错误：sre_constants.error：无需重复

Question

I'm trying to return True only if a letter has a + before and after it

def SimpleSymbols(string): 
if re.search(r"(?<!+)\w(?!+)", string) is None :
    return True
else:
    return False

Answer 1

The unescaped + is a quantifier that repeats the pattern it modifies 1 or more times. To match a literal + , you need to escape it.

However, the (?<!\\+) and (?!\\+) will do the opposite: they will fail the match if a char is preceded or followed with + .

Also, \\w does not match just letters, it matches letters, digits, underscore and with Unicode support in Python 3.x (or with re.U in Python 2.x) even more chars. You may use [^\\W\\d_] instead.

Use

def SimpleSymbols(string): 
    return bool(re.search(r"\+[^\W\d_]\+", string))

It will return True if there is a +[Letter]+ inside a string, or False if there is no match.

See the Python demo :

import re

def SimpleSymbols(string): 
    return bool(re.search(r"\+[^\W\d_]\+", string))
print(SimpleSymbols('+d+dd')) # True
print(SimpleSymbols('ddd'))   # False