Linux Regular Expressions-wildcard characters

Wildcard OFDM character or period (.) is considered to be equivalent to a variable. A variable represents any value in an arithmetic expression. In a regular expression, a period (.) is a wildcard character that represents any character except newline characters (in awk, a period can even match an inline newline character).

Assuming that we are describing a sequence of characters, use the wildcard OFDM character to specify a position where any character can be populated.
For example, if you want to soso a discussion file that contains an Intel system microprocessor, use the following regular expression:
80.86
Will match the containing sequence "80286,", "80386," or "80486." The line. To match a decimal point or period, you must escape the point with a backslash.

It is useless to match only any character at the beginning or end of a pattern. Therefore, a wildcard character is usually placed before or after a character or other meta-character. For example, the following regular expression is written to search for a sequence of chapter:
Chapter.
It searches for "' chapter ' followed by a string with any character". In some searches, this expression may have the same match as a fixed string pattern "chapter". Take a look at the following example:

$ grep' Chapter. 'Sample
You'll find several examplesinchChapter9.
"Quote me ' Chapter and Verse ',"She said.
Chapter Ten

The shown column matches the "chapter" string, and using "chapter" will also match the same line. However, there is a different case where "chapter" appears at the end of the line.

Reference: http://www.linuxawk.com/communication/404.html

Linux Regular Expressions-wildcard characters