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Update and clarify grep regex usage in 05.Rmd #147

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Update and clarify grep regex usage in 05.Rmd #147

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15 changes: 12 additions & 3 deletions book/2e/05.Rmd
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Expand Up @@ -253,13 +253,22 @@ For example, if you only wanted to print the headings that start with *The*:
< alice.txt grep -E '^CHAPTER (.*)\. The'
```

Note that you have to specify the `-E` option in order to enable regular expressions.
Otherwise, `grep` interprets the pattern as a literal string which most likely results in no matches at all:
`-E` is for enabling the ERE (extended regular expression) standard. By default, `grep` uses the BRE (basic regular expression). ERE and BRE support different syntaxes, and each could be the more appropriate choice depending on the situation.

To achieve the same result with BREs, run

```{console}
< alice.txt grep -G '^CHAPTER \(.*\)\. The'
```

This command employs the `-G` flag, indicating the usage of BRE. It is equivalent to:

```{console}
< alice.txt grep '^CHAPTER (.*)\. The'
< alice.txt grep '^CHAPTER \(.*\)\. The'
```

which defaults to using BRE.

With the `-v` option you invert the matches, so that `grep` prints the lines which *don't* match the pattern.
The regular expression below matches lines that contain white space, only.
So with the inverse, and using `wc -l`, you can count the number of non-empty lines:
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