4/20/2023 0 Comments Grep whole word![]() ![]() This example dataset will be used again in ripgrep chapter. While searching huge code bases, you could consider using more than one processing resource (if available) to speed up results. Back-references are very slow, and may require exponential time. In addition, certain other obscure regular expressions require exponential time and space, and may cause grep to run out of memory. $ time LC_ALL=C grep -圎 '() construct may cause grep to use lots of memory. $ # LC_ALL=C will give ASCII locale, active only for this command $ # -w ensures there are no word characters around the matching text $ echo 'I have 12, he has 2!' | grep -o '\' $ # \ strictly match only start and end word boundaries respectively $ echo 'I have 12, he has 2!' | grep -o '\b.\b' ![]() $ # 1st and 3rd line have space as second character $ # \b matches both start and end of word boundaries This should hopefully make it clear the differences between using \b and -w and \ features. The last character ! doesn't have end of word boundary as it is not a word character. $ # matches because $ is preceded by non-word characterĬonsider I have 12, he has 2! as sample text, shown below as image with vertical bars as word boundaries. $ # no output because there are no word characters ![]() This option has no effect if -x is also specified. Word-constituent characters are letters, digits, and the underscore. Similarly, it must be either at the end of the line or followed by a non-word constituent character. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. w, -word-regexp Select only those lines containing matches that form whole words. The \b anchor by definition requires word characters to be present, but this is not the case with -w as described in the manual: The -w option is not exactly the same as using word boundaries in regular expressions. $ printf 'boat\nsite\nfoot' | grep ' t' -E This is useful if you forgot some option(s) and want to edit the previous command from history. $ # escape it (won't work if -F option is also needed)Īs a corollary, you can use options after filename arguments. $ # command assumes - is start of an option, hence the errors This problem and the solution is not unique to the grep command. Either escape it or use - as an option before the pattern to indicate that no more options will be used (handy if pattern is programmatically constructed). Patterns cannot start with - as it will be treated as a command line option. $ # in bash, strings placed next to each other will be concatenated See mywiki.wooledge Quotes for detailed discussion of various quoting and expansions in bash shell. When double quotes are needed, try to use them only for the portion required. $ # will get expanded to: grep -F word_anchors.txt words.txt Always use single quotes for search string/pattern, unless other forms of shell expansion is needed and you know what you are doing. ![]()
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