Escaping char in regex

Question

I ran this example in my bash terminal:

echo "ab" | egrep '\a\b'

Output

ab

then I ran this one:

echo "a" | egrep '\a\b'

Output

I was confused. Why did I get output? But then I input another one:

echo "a" | egrep '\ab'

and didn't get output.

What is the difference between \a\b and \ab for Extended RegEx?

P.S

Regex didn't ignore the first letter:

echo "b" | egrep '\a\b'

Output is empty Regex acted as I expected, but what is the difference from the first case?

P.P.S I found out that this example:

echo 'ab' | egrep '\a\b'

also does not output anything. In the first case (echo "ab" | egrep '\a\b'), the output was ab. How can quotes affect regex?

In grep (GNU grep) 3.1, echo "ab" | egrep '\a\b' does not return any result. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Oct 29, 2020 at 20:16
\b is special, \a is just a more complicated way how to write a. — choroba
– choroba, Commented Oct 29, 2020 at 20:17
Ok, I realized that escaping letters is not safe. But what about the other characters? Do they have other meanings than themselves when escaping? Or is it only letters that have similar properties? — ccaniel
– ccaniel, Commented Oct 29, 2020 at 20:25

Ryszard Czech · Accepted Answer · 2020-10-29 20:35:56Z

The '\a\b' makes \a\b regex where \a is an unknown escape sequence and matches an a and \b is a word boundary.

echo "a" | egrep '\a\b' - correctly outputs a, there is an a not followed with a digit, letter or underscore.

echo "a" | egrep '\ab' - correctly does not output anything, a does not contain ab.

echo "b" | egrep '\a\b' - correctly does not output anything, b does not contain a.

As for echo "ab" | egrep '\a\b', it should not output any result, ab does not contain a at a word boundary, there is b after a.

Other letters that means something else when escaped (GNU grep, POSIX):

Also, there is \< (start of word) and \> (end of word).

If you use -P, PCRE pattern, there are even more escape sequences available (see pcresyntax and pcrepattern).

Ok, I realized that escaping letters is not safe. But what about the other characters? Do they have other meanings than themselves when escaping? Or is it only letters that have similar properties?
Or, escaping without consequences only applies to special characters such as: ^, $, etc. If so, is there some kind of list of all such symbols?

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