I have a string like

text= '{username} joined {servername} {{anytext {find} this {and} this also}} '

and a simple regex code

print(re.findall('{([^{}]*)}', text))

#output
['username', 'servername', 'anytext']

Where anytext is inside double braces but it is also validating by the regex. I mean, the regex should only find word in single braces and ignore double braces.

Please help me to do this happen.

CodePudding user response：

You can match the string against the regular expression

(?<!{{)(?<={)[^{}]*?(?=})(?!}})

Demo

As seen, there are two matches:

'{username} joined {servername} {{anytext}}'
  ^^^^^^^^          ^^^^^^^^^^

The expression can be broken down as follows.

(?<!{{)  # a negative lookbehind asserts the current location
         # in the string is not preceded by '{{' 
(?<={)   # a positive lookbehind asserts the current location
         # in the string is preceded by '{' 
[^{}]*?  # match zero or more characters other than '{' and '}'
(?=})    # a positive lookahead asserts the current location
         # in the string is followed by '}' 
(?!}})   # a negative lookahead asserts the current location
         # in the string is not followed by '}}' 

If desired, the negative lookarounds could be embedded in the positive ones:

(?<=(?<!{{){)[^{}]*?(?=}(?!}))

The string could first be tested to have balanced braces by matching it against the following regular expression.

^[^{}]*(?:(?:{[^{}]*}|{{[^{}]*}})[^{}]*)*$

Demo

Hover the cursor over each part of the expression at the link to obtain an explanation of its function.

CodePudding user response：

Assuming you only expect either {...} or {{...}} we can use the following re.findall trick:

text = '{username} joined {servername} {{anytext}}'
matches = [x for x in re.findall(r'\{\{.*?\}\}|\{(.*?)\}', text) if x]
print(matches)  # ['username', 'servername']

The trick here is to match {{...}} first in the alternation followed by {...}, while only capturing the second single bracket version. We filter off the empty matches, leaving behind the matches we want.