org.apache.lucene.util.automaton

## Class RegExp

- java.lang.Object
- - org.apache.lucene.util.automaton.RegExp

-     public class RegExp
      extends Object

  Regular Expression extension to `Automaton`.

  Regular expressions are built from the following abstract syntax:

  regexp | ::= | unionexp |  |
  unionexp | ::= | interexp `|` unionexp | (union) |
  | | | interexp |  |
  interexp | ::= | concatexp `&` interexp | (intersection) | [OPTIONAL]
  | | | concatexp |  |
  concatexp | ::= | repeatexp concatexp | (concatenation) |
  | | | repeatexp |  |
  repeatexp | ::= | repeatexp `?` | (zero or one occurrence) |
  | | | repeatexp `*` | (zero or more occurrences) |
  | | | repeatexp `+` | (one or more occurrences) |
  | | | repeatexp `{n}` | (`n` occurrences) |
  | | | repeatexp `{n,}` | (`n` or more occurrences) |
  | | | repeatexp `{n,m}` | (`n` to `m` occurrences, including both) |
  | | | complexp |  |
  complexp | ::= | `~` complexp | (complement) | [OPTIONAL]
  | | | charclassexp |  |
  charclassexp | ::= | `[` charclasses `]` | (character class) |
  | | | `[^` charclasses `]` | (negated character class) |
  | | | simpleexp |  |
  charclasses | ::= | charclass charclasses |  |
  | | | charclass |  |
  charclass | ::= | charexp `-` charexp | (character range, including end-points) |
  | | | charexp |  |
  simpleexp | ::= | charexp |  |
  | | | `.` | (any single character) |
  | | | `#` | (the empty language) | [OPTIONAL]
  | | | `@` | (any string) | [OPTIONAL]
  | | | `"` <Unicode string without double-quotes> `"` | (a string) |
  | | | `(` `)` | (the empty string) |
  | | | `(` unionexp `)` | (precedence override) |
  | | | `<` <identifier> `>` | (named automaton) | [OPTIONAL]
  | | | `<n-m>` | (numerical interval) | [OPTIONAL]
  charexp | ::= | <Unicode character> | (a single non-reserved character) |
  | | | `\` <Unicode character> | (a single character) |

  The productions marked [OPTIONAL] are only allowed if specified by the syntax flags passed to the `RegExp` constructor. The reserved characters used in the (enabled) syntax must be escaped with backslash (`\`) or double-quotes (`"..."`). (In contrast to other regexp syntaxes, this is required also in character classes.) Be aware that dash (`-`) has a special meaning in charclass expressions. An identifier is a string not containing right angle bracket (`>`) or dash (`-`). Numerical intervals are specified by non-negative decimal integers and include both end points, and if `n` and `m` have the same number of digits, then the conforming strings must have that length (i.e. prefixed by 0's).

  WARNING: This API is experimental and might change in incompatible ways in the next release.

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `ALL` Syntax flag, enables all optional regexp syntax.
    `static int` | `ANYSTRING` Syntax flag, enables anystring (`@`).
    `static int` | `AUTOMATON` Syntax flag, enables named automata (`<`identifier`>`).
    `static int` | `COMPLEMENT` Syntax flag, enables complement (`~`).
    `static int` | `EMPTY` Syntax flag, enables empty language (`#`).
    `static int` | `INTERSECTION` Syntax flag, enables intersection (`&`).
    `static int` | `INTERVAL` Syntax flag, enables numerical intervals ( `<n-m>`).
    `static int` | `NONE` Syntax flag, enables no optional regexp syntax.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `RegExp(String s)` Constructs new `RegExp` from a string.
    `RegExp(String s, int syntax_flags)` Constructs new `RegExp` from a string.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `Set<String>` | `getIdentifiers()` Returns set of automaton identifiers that occur in this regular expression.
    `String` | `getOriginalString()` The string that was used to construct the regex.
    `Automaton` | `toAutomaton()` Constructs new `Automaton` from this `RegExp`.
    `Automaton` | `toAutomaton(AutomatonProvider automaton_provider, int maxDeterminizedStates)` Constructs new `Automaton` from this `RegExp`.
    `Automaton` | `toAutomaton(int maxDeterminizedStates)` Constructs new `Automaton` from this `RegExp`.
    `Automaton` | `toAutomaton(Map<String,Automaton> automata, int maxDeterminizedStates)` Constructs new `Automaton` from this `RegExp`.
    `String` | `toString()` Constructs string from parsed regular expression.
    `String` | `toStringTree()` Like to string, but more verbose (shows the higherchy more clearly).

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### INTERSECTION

          public static final int INTERSECTION

      Syntax flag, enables intersection (`&`).
      See Also:
        Constant Field Values

    - #### COMPLEMENT

          public static final int COMPLEMENT

      Syntax flag, enables complement (`~`).
      See Also:
        Constant Field Values

    - #### EMPTY

          public static final int EMPTY

      Syntax flag, enables empty language (`#`).
      See Also:
        Constant Field Values

    - #### ANYSTRING

          public static final int ANYSTRING

      Syntax flag, enables anystring (`@`).
      See Also:
        Constant Field Values

    - #### AUTOMATON

          public static final int AUTOMATON

      Syntax flag, enables named automata (`<`identifier`>`).
      See Also:
        Constant Field Values

    - #### INTERVAL

          public static final int INTERVAL

      Syntax flag, enables numerical intervals ( `<n-m>`).
      See Also:
        Constant Field Values

    - #### ALL

          public static final int ALL

      Syntax flag, enables all optional regexp syntax.
      See Also:
        Constant Field Values

    - #### NONE

          public static final int NONE

      Syntax flag, enables no optional regexp syntax.
      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### RegExp

          public RegExp(String s)
                 throws IllegalArgumentException

      Constructs new `RegExp` from a string. Same as `RegExp(s, ALL)`.
      Parameters:
        `s` - regexp string
      Throws:
        `IllegalArgumentException` - if an error occurred while parsing the regular expression

    - #### RegExp

          public RegExp(String s,
                        int syntax_flags)
                 throws IllegalArgumentException

      Constructs new `RegExp` from a string.
      Parameters:
        `s` - regexp string
        `syntax_flags` - boolean 'or' of optional syntax constructs to be enabled
      Throws:
        `IllegalArgumentException` - if an error occurred while parsing the regular expression

  - ### Method Detail

    - #### toAutomaton

          public Automaton toAutomaton()

      Constructs new `Automaton` from this `RegExp`. Same as `toAutomaton(null)` (empty automaton map).

    - #### toAutomaton

          public Automaton toAutomaton(int maxDeterminizedStates)
                                throws IllegalArgumentException,
                                       TooComplexToDeterminizeException

      Constructs new `Automaton` from this `RegExp`. The constructed automaton is minimal and deterministic and has no transitions to dead states.
      Parameters:
        `maxDeterminizedStates` - maximum number of states in the resulting automata. If the automata would need more than this many states TooComplextToDeterminizeException is thrown. Higher number require more space but can process more complex regexes.
      Throws:
        `IllegalArgumentException` - if this regular expression uses a named identifier that is not available from the automaton provider
        `TooComplexToDeterminizeException` - if determinizing this regexp requires more than maxDeterminizedStates states

    - #### toAutomaton

          public Automaton toAutomaton(AutomatonProvider automaton_provider,
                                       int maxDeterminizedStates)
                                throws IllegalArgumentException,
                                       TooComplexToDeterminizeException

      Constructs new `Automaton` from this `RegExp`. The constructed automaton is minimal and deterministic and has no transitions to dead states.
      Parameters:
        `automaton_provider` - provider of automata for named identifiers
        `maxDeterminizedStates` - maximum number of states in the resulting automata. If the automata would need more than this many states TooComplextToDeterminizeException is thrown. Higher number require more space but can process more complex regexes.
      Throws:
        `IllegalArgumentException` - if this regular expression uses a named identifier that is not available from the automaton provider
        `TooComplexToDeterminizeException` - if determinizing this regexp requires more than maxDeterminizedStates states

    - #### toAutomaton

          public Automaton toAutomaton(Map<String,Automaton> automata,
                                       int maxDeterminizedStates)
                                throws IllegalArgumentException,
                                       TooComplexToDeterminizeException

      Constructs new `Automaton` from this `RegExp`. The constructed automaton is minimal and deterministic and has no transitions to dead states.
      Parameters:
        `automata` - a map from automaton identifiers to automata (of type `Automaton`).
        `maxDeterminizedStates` - maximum number of states in the resulting automata. If the automata would need more than this many states TooComplexToDeterminizeException is thrown. Higher number require more space but can process more complex regexes.
      Throws:
        `IllegalArgumentException` - if this regular expression uses a named identifier that does not occur in the automaton map
        `TooComplexToDeterminizeException` - if determinizing this regexp requires more than maxDeterminizedStates states

    - #### getOriginalString

          public String getOriginalString()

      The string that was used to construct the regex. Compare to toString.

    - #### toString

          public String toString()

      Constructs string from parsed regular expression.
      Overrides:
        `toString` in class `Object`

    - #### toStringTree

          public String toStringTree()

      Like to string, but more verbose (shows the higherchy more clearly).

    - #### getIdentifiers

          public Set<String> getIdentifiers()

      Returns set of automaton identifiers that occur in this regular expression.

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