![]() In fact, the official standard for BRE doesn't even support the '+' quantifier! This can lead to very confusing behaviour since you might just try escaping the '+' and find that it gives you the result you expect: grep " Hello. If you don't specify any flags to grep, it will use 'BRE' or 'Basic Regular Expressions' which are very old and quite primitive. Since you know that '*' is a regex pattern for 'zero or more' and '+' is a regex pattern for 'one or more', you decide to try the following: grep " Hello.+World " hello.txtīut this doesn't match anything at all! What's going on here? Isn't '+' a regular expression symbol for 'one or more'? The answer is related to the default regular expression mode that grep uses. You could use a grep command like this: grep " Hello.*World " hello.txtĪnd as you expect, it finds these matches: Hello World.īut now you might consider adding the additional requirement that there be at least one character between 'Hello' and 'World' so that the line with 'HelloWorld' is not included in the matches. ![]() Let's say that you wanted to use the 'grep' command to find all lines in this file that contain the word 'Hello' followed by the word 'World'. Here is a file containing a few lines of text that we'll place inside the file 'hello.txt': Hello World. Therefore, if you try to do a grep search like this, you'll be waiting a long time (forever). In other words, it's waiting for you to type in some text by hand so it can search it for your term. In this case, grep will just sit there and do nothing because it's waiting for you to type some input (until CTRL+d is pressed) since nothing was fed in from stdin. Or, directly from stdin like this when no files are specified: echo " something " | grep " test "Ī common mistake is to forget to specify a file name at all when issuing a grep search when there is no input from stdin: grep " test " The grep command can accept input from two different methods: 1) From one or more files, like this: grep " test " file.txt (Added on ) By popular demand, many people suggested that I add this item as #0 to this list. The root causes of these mistakes are: Not knowing what flavour of regular expression that grep is currently using (and/or not understanding what features that flavour supports) Not considering the escaping rules of your shell Issues with character encodings. I have chosen to document these mistakes in detail because they are mistakes that beginners are very likely to make at some point, but also to not be able to debug on their own. In this article, I'll discuss 5 very confusing mistakes that have caused me to waste significant amounts of time when using the command-line tool known as 'grep' to search for things. The Most Confusing Grep Mistakes I've Ever Made - By Robert Elder Introduction
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