By default, regular expressions are considered greedy. This might seem a strange term to describe a technical concept, but it does fit really well. To illustrate why regular expressions are considered greedy, look at this example:

reader@ubuntu:~/scripts/chapter_10$ grep 'in' grep-file.txt 
We can use this regular file for testing grep.
Did you ever realise that in the UK they say colour,
but in the USA they use color (and realize)!
reader@ubuntu:~/scripts/chapter_10$ grep 'the' grep-file.txt 
Did you ever realise that in the UK they say colour,
but in the USA they use color (and realize)!

As you can see, grep does not by default look for full words. It looks at the characters in the file, and if a string matches the search (regardless of what comes before or after them), the line is printed.

In the first example, in matches both the normal word in, but also testing. In the second example, both lines have two matches, both the and they.

If you want to return whole words only, be sure to include the spaces in your grep search pattern:

reader@ubuntu:~/scripts/chapter_10$ grep ' in ' grep-file.txt 
Did you ever realise that in the UK they say colour,
but in the USA they use color (and realize)!
reader@ubuntu:~/scripts/chapter_10$ grep ' the ' grep-file.txt 
Did you ever realise that in the UK they say colour,
but in the USA they use color (and realize)!

As you can see, the search for ' in ' now does not return the line with the word testing, since the string of characters in isn't surrounded by spaces there.