NFD » ndn-cxx

# NDN Regular Expression

NDN regular expression matching is done at two levels: one at the name level and one at the name component level.

We use `<` and `>` to enclose a name component matcher which specifies the pattern of a name component.

The component pattern is expressed using the [Perl Regular Expression Syntax](http://www.boost.org/doc/libs/1_55_0/libs/regex/doc/html/boost_regex/syntax/perl_syntax.html).

For example, `<ab*c>` can match the 1st, 3rd, and 4th components of `"/ac/dc/abc/abbc"`, but it cannot match the 2nd component.

A special case is that `<>` is a wildcard matcher that can match **ANY** component.

Note that a component match can match only one name component.

In order to match a name, you need to specify the pattern of a name based on the name component matchers.

For example, `<ndn><edu><ucla>` can match the name `"/ndn/edu/ucla"`.

In order to describe a more complicated name pattern, we borrow some syntaxes from the standard regular expressions.

## NDN Regex Syntax

### Anchors

A `'^'` character shall match the start of a name.

For example, `^<ndn>` shall match any names starting with a component `"ndn"`, and it will exclude a name like `"/local/broadcast"`.

A `'$'` character shall match the end of a name.

For example, `^<ndn><edu>$` shall match only one name: `"/ndn/edu"`. 

A NDN regular expression is built on the component matcher extended from the standard regex by simply treating a component matcher as a single character in standard regex.

For example, 

`^<ndn>` can match any names starting with a component `"ndn"`; 

`<test>$` can match any names ending with a component `"test"`;

`^[^<ndn>]` can match any names that do not start with a component `"ndn"`;

`^([^<DNS>])<DNS>(<>*)<NS>` can match a NDN DNS data name, and you can use back reference to extract the part enclosed by `(` and `)`

(`"\1\2"` can extract `/ndn/edu/ucla/irl` from `/ndn/edu/ucla/DNS/irl/NS/123456`.