Regex » History » Revision 6
Revision 5 (Yingdi Yu, 03/19/2014 12:00 PM) → Revision 6/7 (Alex Afanasyev, 12/16/2015 10:25 PM)
# NDN Regular Expression Documentation for NDN regular expression has been moved matching is done at two levels: one at the name level and one at the name component level. We use `<` and `>` to [ndn-cxx 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 expressions tutorial](http://named-data.net/doc/ndn-cxx/current/tutorials/utils-ndn-regex.html) 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.