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I have been researching this a lot, but I am still not clear on it. What does word boundary mean? What does it do?

So, for example, could someone explain this command to me please?

egrep '\b[A-Z]+\b' filename.sh
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A "word boundary" is whatever the tool used defines as such... some consider nospace/space or space/nospace a word boundary, others consider a "word" as composed of [azA-Z0-9_]. Check the manual for egrep(1), perhaps the doumentation for the regular expressions in use. – vonbrand Apr 7 '13 at 1:10

As described here, for example, it matches between words:

There are three different positions that qualify as word boundaries:

  1. Before the first character in the string, if the first character is a word character.
  2. After the last character in the string, if the last character is a word character.
  3. Between two characters in the string, where one is a word character and the other is not a word character.

Here are examples of each of those cases:

  1. For the string foobar, the first case matches

  2. For the string foobar, the second case matches

  3. For the string foo bar, the third case will match

    foo bar
       ^--here, because space is not a word character

What qualifies as a word character depends on the specific regular expression implementation. In all cases however, letters ([a-z] and [A-Z]), numbers ([0-9]) and _ are considered word characters.

So, the example regex you posted (\b[A-Z]+\b) means find the longest string that is between two word boundaries and that consists of upper case letters only. It might be easier to explain by example:

echo "FOOBAR" | egrep '\b[A-Z]+\b'   # Works
echo "FOO BAR" | egrep '\b[A-Z]+\b'  # Works
echo "aFOOBARb" | egrep '\b[A-Z]+\b' # Does not work, we want capitals only
echo "12345" | egrep '\b[A-Za]+\b'   # Does not work, no letters
echo "1FOOBAR2" | egrep '\b[A-Z]+\b' # Does not work, 1 and 2 are word chars
echo "_FOOBAR_" | egrep '\b[A-Z]+\b' # Does not work, _ is a word char
echo "#FOOBAR$" | egrep '\b[A-Z]+\b' # Works, # and $ are not word chars
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