Character Sets - Arangodb

| | Character Sets | |

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The character set chset matches a set of characters over a finite range bounded by the limits of its template parameter CharT. This class is an optimization of a parser that acts on a set of single characters. The template class is parameterized by the character type CharT and can work efficiently with 8, 16 and 32 and even 64 bit characters.

template \<typename CharT = char\> class chset;

The chset is constructed from literals (e.g. 'x'), ch_p or chlit<>, range_p or range<>, anychar_p and nothing_p (see primitives) or copy-constructed from another chset. The chset class uses a copy-on-write scheme that enables instances to be passed along easily by value.

| Sparse bit vectors

To accommodate 16/32 and 64 bit characters, the chset class statically switches from a std::bitset implementation when the character type is not greater than 8 bits, to a sparse bit/boolean set which uses a sorted vector of disjoint ranges (range_run). The set is constructed from ranges such that adjacent or overlapping ranges are coalesced.

range_runs are very space-economical in situations where there are lots of ranges and a few individual disjoint values. Searching is O(log n) where n is the number of ranges. |

Examples:

chset\<\> s1('x'); chset\<\> s2(anychar\_p - s1);

Optionally, character sets may also be constructed using a definition string following a syntax that resembles posix style regular expression character sets, except that double quotes delimit the set elements instead of square brackets and there is no special negation ^ character.

range = anychar\_p \>\> '-' \>\> anychar\_p; set = \*(range\_p | anychar\_p);

Since we are defining the set using a C string, the usual C/C++ literal string syntax rules apply. Examples:

chset\<\> s1("a-zA-Z"); // alphabetic characters chset\<\> s2("0-9a-fA-F"); // hexadecimal characters chset\<\> s3("actgACTG"); // DNA identifiers chset\<\> s4("\x7f\x7e"); // Hexadecimal 0x7F and 0x7E

The standard Spirit set operators apply (see operators) plus an additional character-set-specific inverse (negation ~) operator:

| Character set operators | | ~a | Set inverse | | a | b | Set union | | a & | Set intersection | | a - b | Set difference | | a ^ b | Set xor |

where operands a and b are both chsets or one of the operand is either a literal character, ch_p or chlit, range_p or range, anychar_p or nothing_p. Special optimized overloads are provided for anychar_p and nothing_p operands. A nothing_p operand is converted to an empty set, while an anychar_p operand is converted to a set having elements of the full range of the character type used (e.g. 0-255 for unsigned 8 bit chars).

A special case is ~anychar_p which yields nothing_p, but ~nothing_p is illegal. Inversion of anychar_p is asymmetrical, a one-way trip comparable to converting T* to a void*.

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Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE\_1\_0.txt)