7.8.2. TokenBigram

7.8.2.1. Summary

TokenBigram is a bigram based tokenizer. It’s recommended to use this tokenizer for most cases.

Bigram tokenize method tokenizes a text to two adjacent characters tokens. For example, Hello is tokenized to the following tokens:

  • He
  • el
  • ll
  • lo

Bigram tokenize method is good for recall because you can find all texts by query consists of two or more characters.

In general, you can’t find all texts by query consists of one character because one character token doesn’t exist. But you can find all texts by query consists of one character in Groonga. Because Groonga find tokens that start with query by predictive search. For example, Groonga can find ll and lo tokens by l query.

Bigram tokenize method isn’t good for precision because you can find texts that includes query in word. For example, you can find world by or. This is more sensitive for ASCII only languages rather than non-ASCII languages. TokenBigram has solution for this problem described in the below.

7.8.2.2. Syntax

TokenBigram hasn’t parameter:

TokenBigram

7.8.2.3. Usage

TokenBigram behavior is different when it’s worked with any Normalizers.

If no normalizer is used, TokenBigram uses pure bigram (all tokens except the last token have two characters) tokenize method:

Execution example:

tokenize TokenBigram "Hello World"
# [
#   [
#     0,
#     1337566253.89858,
#     0.000355720520019531
#   ],
#   [
#     {
#       "position": 0,
#       "force_prefix": false,
#       "value": "He"
#     },
#     {
#       "position": 1,
#       "force_prefix": false,
#       "value": "el"
#     },
#     {
#       "position": 2,
#       "force_prefix": false,
#       "value": "ll"
#     },
#     {
#       "position": 3,
#       "force_prefix": false,
#       "value": "lo"
#     },
#     {
#       "position": 4,
#       "force_prefix": false,
#       "value": "o "
#     },
#     {
#       "position": 5,
#       "force_prefix": false,
#       "value": " W"
#     },
#     {
#       "position": 6,
#       "force_prefix": false,
#       "value": "Wo"
#     },
#     {
#       "position": 7,
#       "force_prefix": false,
#       "value": "or"
#     },
#     {
#       "position": 8,
#       "force_prefix": false,
#       "value": "rl"
#     },
#     {
#       "position": 9,
#       "force_prefix": false,
#       "value": "ld"
#     },
#     {
#       "position": 10,
#       "force_prefix": false,
#       "value": "d"
#     }
#   ]
# ]

If normalizer is used, TokenBigram uses white-space-separate like tokenize method for ASCII characters. TokenBigram uses bigram tokenize method for non-ASCII characters.

You may be confused with this combined behavior. But it’s reasonable for most use cases such as English text (only ASCII characters) and Japanese text (ASCII and non-ASCII characters are mixed).

Most languages consists of only ASCII characters use white-space for word separator. White-space-separate tokenize method is suitable for the case.

Languages consists of non-ASCII characters don’t use white-space for word separator. Bigram tokenize method is suitable for the case.

Mixed tokenize method is suitable for mixed language case.

If you want to use bigram tokenize method for ASCII character, see TokenBigramSplitXXX type tokenizers such as TokenBigramSplitSymbolAlpha.

Let’s confirm TokenBigram behavior by example.

TokenBigram uses one or more white-spaces as token delimiter for ASCII characters:

Execution example:

tokenize TokenBigram "Hello World" NormalizerAuto
# [
#   [
#     0,
#     1337566253.89858,
#     0.000355720520019531
#   ],
#   [
#     {
#       "position": 0,
#       "force_prefix": false,
#       "value": "hello"
#     },
#     {
#       "position": 1,
#       "force_prefix": false,
#       "value": "world"
#     }
#   ]
# ]

TokenBigram uses character type change as token delimiter for ASCII characters. Character type is one of them:

  • Alphabet
  • Digit
  • Symbol (such as (, ) and !)
  • Hiragana
  • Katakana
  • Kanji
  • Others

The following example shows two token delimiters:

  • at between 100 (digits) and cents (alphabets)
  • at between cents (alphabets) and !!! (symbols)

Execution example:

tokenize TokenBigram "100cents!!!" NormalizerAuto
# [
#   [
#     0,
#     1337566253.89858,
#     0.000355720520019531
#   ],
#   [
#     {
#       "position": 0,
#       "force_prefix": false,
#       "value": "100"
#     },
#     {
#       "position": 1,
#       "force_prefix": false,
#       "value": "cents"
#     },
#     {
#       "position": 2,
#       "force_prefix": false,
#       "value": "!!!"
#     }
#   ]
# ]

Here is an example that TokenBigram uses bigram tokenize method for non-ASCII characters.

Execution example:

tokenize TokenBigram "日本語の勉強" NormalizerAuto
# [
#   [
#     0,
#     1337566253.89858,
#     0.000355720520019531
#   ],
#   [
#     {
#       "position": 0,
#       "force_prefix": false,
#       "value": "日本"
#     },
#     {
#       "position": 1,
#       "force_prefix": false,
#       "value": "本語"
#     },
#     {
#       "position": 2,
#       "force_prefix": false,
#       "value": "語の"
#     },
#     {
#       "position": 3,
#       "force_prefix": false,
#       "value": "の勉"
#     },
#     {
#       "position": 4,
#       "force_prefix": false,
#       "value": "勉強"
#     },
#     {
#       "position": 5,
#       "force_prefix": false,
#       "value": "強"
#     }
#   ]
# ]