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Consider the following example:
> testLines <- c("I don't want to match this","This is what I want to match") > grepl('is',testLines) > [1] TRUE TRUE What I want, though, is to only match 'is' when it stands alone as a single word. From reading a bit of perl documentation, it seemed that the way to do this is with \b, an anchor that can be used to identify what comes before and after the patter, i.e. \bword\b matches 'word' but not 'sword'. So I tried the following example, with use of Perl syntax set to 'TRUE':
> grepl('\bis\b',testLines,perl=TRUE) > [1] FALSE FALSE The output I'm looking for is FALSE TRUE.
972
The grepl() stands for “grep logical”. In R it is a built-in function that searches for matches of a string or string vector. The grepl() method takes a pattern and data and returns TRUE if a string contains the pattern, otherwise FALSE.
To run a “whole words only” search using a regular expression, simply place the word between two word boundaries, as we did with ‹ \bcat\b ›. The first ‹ \b › requires the ‹ c › to occur at the very start of the string, or after a nonword character.
Basically (0+1)* mathes any sequence of ones and zeroes. So, in your example (0+1)*1(0+1)* should match any sequence that has 1. It would not match 000 , but it would match 010 , 1 , 111 etc. (0+1) means 0 OR 1.
Most characters, including all letters ( a-z and A-Z ) and digits ( 0-9 ), match itself. For example, the regex x matches substring "x" ; z matches "z" ; and 9 matches "9" . Non-alphanumeric characters without special meaning in regex also matches itself. For example, = matches "=" ; @ matches "@" .
"\<" is another escape sequence for the beginning of a word, and "\>" is the end. In R strings you need to double the backslashes, so:
> grepl("\\<is\\>", c("this", "who is it?", "is it?", "it is!", "iso")) [1] FALSE TRUE TRUE TRUE FALSE Note that this matches "is!" but not "iso".
155
you need double-escaping to pass escape to regex:
> grepl("\\bis\\b",testLines) [1] FALSE TRUE If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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