8/23/2023 0 Comments Grep wildcard operatorAll of these operators are PostgreSQL-specific. There are also !~~ and !~~* operators that represent NOT LIKE and NOT ILIKE, respectively. The operator ~~ is equivalent to LIKE, and ~~* corresponds to ILIKE. This is not in the SQL standard but is a PostgreSQL extension. The key word ILIKE can be used instead of LIKE to make the match case-insensitive according to the active locale. PostgreSQL's behavior in this regard is therefore slightly nonstandard. This effectively disables the escape mechanism, which makes it impossible to turn off the special meaning of underscore and percent signs in the pattern.Īccording to the SQL standard, omitting ESCAPE means there is no escape character (rather than defaulting to a backslash), and a zero-length ESCAPE value is disallowed. It's also possible to select no escape character by writing ESCAPE ''. See Section 4.1.2.1 for more information. If you have standard_conforming_strings turned off, any backslashes you write in literal string constants will need to be doubled. To match the escape character itself, write two escape characters. The default escape character is the backslash but a different one can be selected by using the ESCAPE clause. To match a literal underscore or percent sign without matching other characters, the respective character in pattern must be preceded by the escape character. Therefore, if it's desired to match a sequence anywhere within a string, the pattern must start and end with a percent sign. LIKE pattern matching always covers the entire string. An underscore ( _) in pattern stands for (matches) any single character a percent sign ( %) matches any sequence of zero or more characters. If pattern does not contain percent signs or underscores, then the pattern only represents the string itself in that case LIKE acts like the equals operator. An equivalent expression is NOT ( string LIKE pattern).) (As expected, the NOT LIKE expression returns false if LIKE returns true, and vice versa. The LIKE expression returns true if the string matches the supplied pattern. The returned object is a regular expression.9.7.1. If you want to match "blue*" where * has the usual wildcard, not regular expression, meaning we use glob2rx() to convert the wildcard pattern into a useful regular expression: > glob2rx("blue*") You then need to pass this regular expression onto one of R's pattern matching tools. Glob2rx() converts a pattern including a wildcard into the equivalent regular expression. Anybody reading this should now scroll down to Gavin's answer and upvote it.) (I did understand the scoping issues that were ar the root of the questioner's difficulties. I don't think I actually understood glob2rx until I came back to this question. If you want to use subset then consider the closely related function grepl() which returns a vector of logicals can be used in the subset argument: subset(a, grepl("blue", a$x))Įdit: Adding one "proper" use of glob2rx within subset(): result <- subset(a, grepl(glob2rx("blue*"), x) ) Try using grep() which is the workhorse function for pattern matching of character vectors: result <- a # Note need to use `a$` to get at the `x` Rownames or elements inside such objects are not visible to ls() (unless of course you add an environment argument to the ls(.)-call). If you want to examine elements inside a dataframe you should not be using ls() which only looks at the names of objects in the current workspace (or if used inside a function in the current environment).
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