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# 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`. 

### Repeats

A component matcher can be followed by a repeat syntax to indicate how many times the preceding component can be matched.

Syntax `*` for zero or more times.

For example, `^<ndn><KEY><>*<ID-CERT>` shall match `/ndn/KEY/ID-CERT/`, or `/ndn/KEY/edu/ID-CERT`, or `/ndn/KEY/edu/ksk-12345/ID-CERT` and so on.

Syntax `+` for one or more times.

For example, `^<ndn><KEY><>+<ID-CERT>` shall match `/ndn/KEY/edu/ID-CERT`, or `/ndn/KEY/edu/ksk-12345/ID-CERT` and so on, but it cannot match `/ndn/KEY/ID-CERT/`.

Syntax `?` for zero or one times.

For example, `^<ndn><KEY><>?<ID-CERT>` shall match `/ndn/KEY/ID-CERT/`, or `/ndn/KEY/edu/ID-CERT`, but it cannot match `/ndn/KEY/edu/ksk-12345/ID-CERT`.

Repetition can also be bounded:

`{n}` for exactly `n` times. `{n,}` for at least `n` times. `{,n}` for at most `n` times. And `{n, m}` for `n` to `m` times.

Note that the repeat matching is **greedy**, 

that is it will consume as many matched components as possible.

We do not support non-greedy repeat matching and possessive repeat matching for now.

### Sets

Name component set is a bracket-expression starting with `'['` and ending with `']'`, 

it defines a set of name components, and matches any single name component that is a member of that set.

Unlike the standard regular expression, 

NDN regular expression only supports **Single Components Set**, 

that is, you have to list all the set members one by one between the bracket.

For example, `^[<ndn><localhost>]` shall match any names starting with either a component `ndn"` or `localhost`.

When a name component set starts with a `'^'`, the set becomes a **Negation Set**, 

that is, it matches the complement of the name components it contains.

For example, `^[^<ndn>]` shall match any names that does not start with a component `ndn`.

Some other types of sets, such as Range Set, will be supported later.

Note that component set can be repeated as well.

### Sub-pattern and Back Reference

A section beginning `(` and ending `)` acts as a marked sub-pattern. 

Whatever matched the sub-pattern is split out in a separate field by the matching algorithms. 

For example `^([^<DNS>])<DNS>(<>*)<NS>` shall match a data name of NDN DNS NS record, 

and the first sub-pattern captures the zone name while the second sub-pattern captures the relative record name.

Marked sub-patterns can be referred to by a back-reference `\n`.

The same example above shall match a name `/ndn/edu/ucla/DNS/irl/NS/123456`, 

and a back reference `\1\2` shall extract `/ndn/edu/ucla/irl` out of the name.

Note that marked sub-patterns can be also repeated.