正则表达式和python表示不匹配多个连续字符

Question

I know in regex we can use ^ to declare something except. For example [^ ]*? means a string with no space. How we can use this to find the except for more than two consecutive character. Fro example a string that doesn't contain {{ when it can contain a single { . I tried these and didn't work:

re.compile(r"(\{\{`[^(\{\{)]*?\}\}`)
re.compile(r"(\{\{`[^\{\{]*?\}\}`)

This is to catch strings in a file that starts with {{ and ends with }} but doesn't contains }} while they can contain a single } . Also using .* is not an option.

input_string="blah blah blah {{cite journal |last=Malatesta|first=Errico|title=Towards Anarchism|journal=MAN!|publisher=International Group of San Francisco|location=Los Angeles|oclc=3930443|url=http://www.marxists.org/archive/malatesta/1930s/xx/toanarchy.htm|archiveurl=http://web.archive.org/web/20121107221404/http://marxists.org/archive/malatesta/1930s/xx/toanarchy.htm|archivedate=7 November 2012 |deadurl=no|authorlink=Errico Malatesta |ref=harv}} blah blah blah"
regexp_1 = re.compile(r"(\{\{[^\}]*?\}\})") 
output = regexp_1.sub("",input_string )

Now regexp_1 , I want to replace [^\\}]*? with [^\\}\\}]*? and I know that [^\\}\\}]*? is not correct since it works the same way as [^\\}]*? .

Answer 1

This is to catch strings in a file that starts with {{ and ends with }} but doesn't contains }} while they can contain a single }

your_string = "{{first group}} {{second {} group}}"
pattern = re.compile(r'{{.*?}}')
pattern.findall(your_string)  # returns list of matches 

Which will return

['{{first group}}', '{{second {} group}}']

Answer 2

It looks like what you actually want is to match first }} after {{. The easiest regexp which will do this is:

\{\{.*?\}\}

Make sure to configure . to match line breaks if you allow them to be inside.

If you concerned about performance I would say that this regexp is one of the fastest one. Alternatives would be:

1) Use negative lookahead

\{\{((?!\}\}).)*\}\}

Have comparable performance as you will have look ahead check for every character

2) Use atomic group and possessive quantifier

\{\{(?>[^{]|\{[^{])**\}\}

This one might actually be faster as due to use of "?>" and "**" construction it won't dive up already matched values - so will do everything with single run. PS: make sure your regexp engine supports this constructions.

Answer 3

For that case you can use a negative look ahead :

^((?!}}).)*$

And for catching the string between {{ and }} you can use re.search() with aforementioned regex.

>>> s = 'this {{ is {a} sample }}text'
>>> re.search(r'{{(((?!}}).)*)}}',s).group(1)
' is {a} sample '

Answer 4

As far as I know, you can't use something like [^word] since this will only match whatever character but w , o , r , d .

Also I know you can use negative lookaheads like myword(?!something) to match myword only if it is not followed with something .

However, to match something that is not a word I know you have to use some tricks like what is described in this post Match everything except for specified strings

For your specific case, you can use this regex to check if the line contains {{ :

^(?!.*\{\{)

Regex Demo

On the other hand, if you use PCRE regex then you can use the discard verbs, so if you want to skip patterns like {{something}} , you can use this:

\{\{\w+\}\}(*SKIP)(*FAIL)|(\w+)
           ^^^^^^^^^^^^^^ if your pattern matches, it will be discarded intentionally 

Working demo