Regular expressions - character classes, construction of choice, metasequence

In J. Friedl's book “Regular expressions” there is a beautiful sign I want to give here.
Regular expressions provide ample opportunities for search and replace in any text. Using regular expressions you can flexibly and simply process text documents. One of the simplest uses of regular expressions is text search - many text editors have the ability to search for regular expression patterns . There are several types of metacharacters that perform different functions in regexp , let's briefly look at them:

• ^ and $ - “tie” the matches of the rest of the regular expression to the beginning and end of the string, respectively. For example, ^cat matches the strings "cat" and "caterpillar", and dog$ with the strings "bulldog" and "hotdog".
• […] - character classes allow you to list characters that can be in a given position of the text. For example, gr[ea]y will match the strings “gray” and “gray”.
• [^…] - excluding character classes allow you to list characters that can not be in a given position of the text. For example, g[^ae]rdy does not match the strings "gardy" and "gerdy", but will coincide with the strings "gurdy", "g3rdy" and "girdy".
• (…|…) - design choice , the choice of several options. It should be noted that each part of the construction of choice is a full-fledged regular expression . For example, Jeff(re|er)y matches the strings “Jeffrey” and “Jeffery”.
• \< and \> - metasequences are an analogue of ^ and $ but at the level of words. For example, \<cat\> matches the single word cat.

It should be noted that the rules that determine the composition of supported metacharacters (and their interpretation) are completely different inside and outside of character and excluding character classes.

Regex metacharacters
Symbol	Title	Interpretation
Elements denoting a single character
.	point	one any character
[…]	character class	any of the listed characters
[^…]	inverted character class	any character not listed in class
\	screening	if the character is preceded by the escape prefix "\", then the character is interpreted as the corresponding literal
Quantifiers
?	question mark	one copy is allowed (none required)
*	star	any number of copies allowed (none required)
+	a plus	one copy is required, any number of copies allowed
{min, max}	interval quantifier *	“min” instances required, “max” results allowed
Positional metacharacters
^	cover, circumflex	position at the beginning of the line
$	dollar	position at the end of the line
\<	word boundary *	position at the beginning of the word
\>	word boundary *	position at the end of a word
Other metacharacters
|	selection design	any of the listed expressions
(…)	round brackets	constraint for choice construction, grouping for applying quantifiers, and saving text for backlinks
\1, \2, …	back link	text that previously matched the first, second, etc. pairs of parentheses

^* these features are not supported by all egrep versions