What does this regex do?
I am working on converting a program from Perl to Java. I ran into the line
my ($title) = ($info{$host} =~ /^\s*\(([^\)]+)\)\s*$/);
I am not very good at regex, but from what I can tell it is matching something in the $ info {$ host} line with a regex ^ \ s * (([^)] +)) \ s * $ and assigning the match to $ title.
My problem is that I don't know what the regex is doing and what it will match. Any help would be appreciated.
thank
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The regular expression matches a string containing exactly one pair of matching parentheses (actually, one opening and one matching closing parenthesis, but within any number of additional opening parentheses).
A string can begin and end with whitespace characters, but not others. However, there can be arbitrary characters (at least one) in parentheses.
The following lines should match it:
(abc) (() (ab)
By the way, you can just use as-is regex in Java (after exiting backslashes) using the Pattern
class.
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Ok step by step
/ - quote regex
^ - beginning of line
\ s * - zero or more of any spacelike character
(- actual (symbol
(- start a capture group
[^)] + any of the characters ^ or) + specifying at least one
) - accept capture group
) and the actual) character
\ s * zero or more space as characters
$ - end of line
/ - close quote from regex
So, as far as I can understand this, we are looking for strings like "(^)" or "())" I think there is something missing here.
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my ($title) = ($info{$host} =~ /^\s*\(([^\)]+)\)\s*$/);
First, m//
in the context of a list, it returns the committed matches. my ($title)
puts the right side in the context of the list. Second, it $info{$host}
matches the following pattern:
/^ \s* \( ( [^\)]+) \) \s* $/x
Yes, used a flag x
so I could insert some spaces. ^\s*
skips all leading spaces. Then we have an escaped paratet (so no capturing group is created, then we have a capturing group containing [^\)]
. This character class is better written as [^)]
, because the correct parenthesis is not special in the character class and means something but a left parenthesis.
If there are one or more characters other than a closing parenthesis following an opening parenthesis followed by a closing parenthesis that is optionally surrounded on each side by a space, that sequence of characters is captured and placed in $x
.
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