I think that I figure it out now, but it took me a while to google and digest the related information. I think it is worthwhile to study this. I am writing this blog to summarize my findings.
Lookahead and Lookbehand Zero-width Assertions
At first I thought about match a pattern not containing another pattern should be as simple as using a negate or ! operator to identify not-containing-pattern. There may be a not operator in VIM search, but I could not find it. What I found is Lookaround Zero-width Assertion.
In VIM, the way of search for a pattern not containing another pattern is very smart and elegant. The basic search is to find all matched patterns, and the matched items are returned as results. In VIM, the following is a search command:
/PATTERNthe pattern can be a regular expression.
In VIM,
zero width pattern is a pattern to be matched but not in the search results. Think zero width pattern as additional match condition, it can be described as either of following ways:PATTREN + ZERO_WIDTH_ATOM orZERO_WIDTH_ATOM + PATTERNThe first one is called as lookahead zero width assertion, and second one as lookbehind zero width assertion. Assertion here means matched or not matched. The above two are positive assertions. If we take negative or not matched into consideration, there are four types of look around with zero width assertions. They are:
/PATTERN[ZERO_WIDTH_ATOM\@=]/PATTERN[ZERO_WIDTH_ATOM\@!]/[ZERO_WIDTH_ATOM\@<=]PATTERN/[ZERO_WIDTH_ATOM\@<!]PATTERNNote: [...] is used as optional and also as separator from pattern, [ or ] are not part of search.
As my understanding, VIM uses symbolic like character for look around. As other special characters in VIM, \ is used to indicate look around with zero width assertion. The following table summarizes characters used for this type of search:
| \@ | indicate lookahead |
| \@< | lookbehind |
| = | positive match |
| ! | negative or not match. |
In above searches,
PATTERN is to be matched. If ZERO_WIDTH_ATOM is supplied, it will be used as additional assertion. If there is any match, the matched pattern items will be returned as results, but ZERO_WIDTH_ATOM is not in the results. That's why it called as zero width.According to VIM documentation, the definition of ATOM is a character, or a character class, or a group (indicated by \(...\) braces).
Think the Search as a Program
Lets think those type of searches as a program. Here I have the following c-style pseudo codes:
Results getMatchedResults( string context, // basic string pattern, string zero_width_pattern, // zero_width pattern bool match_zero_width_pattern, bool lookahead) { results = EMPTY_LIST; result = getMatchedResult(context, pattern); while (result != EMPTY) { if (zero_pattern != EMPTY) { if ( lookahead ) { context_tmp = getNextContextByLookahead(context, result); result_tmp = getMatchedResult(context_tmp, zero_width_pattern); } else { context_tmp = getNextContextByLookbehind(context, result); result_tmp = getMatchedResultByLookbehind(context_tmp, zero_width_pattern); } if ( result_tmp == EMPTY ) { if (match_zero_width_pattern) { result = EMPTY; }
} else { if (!match_zero_width_pattern) { retult = EMPTY; } } } if ( result != EMPTY ) { results.add(result); context = geNextContextByLookahread(context, result); result = getMatchedResult(context, pattern); } } return results; }
The pseudo codes are very straightforward. Actually, VIM search is based on Regex as its search engine. The above search expression commands are basic Regex patterns.
References
- VIM Tip: Not Containing Pattern (1)
- VIM Doc: pattern and zero width
- René article on VIM Search
- Regular-Expression.Info: Lookahead and Lookbehind Assertions
- Next blog: VIM Tip: Not Containing Pattern (3)
I was suppressed to know the web service of Wolfram|Alpha, a computable knowledge engine. I knew this one by aan article of AppleInsider about 1/4 Siri searching result from Wolfram|Alpha.
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