Common RegEx Expression

Allow a-zA-Z0-9 ( no space)	^[a-zA-Z0-9]+$
Allow a-zA-Z0-9 and space ( look at \s)	^[a-zA-Z0-9\s]+$
Allow a-zA-Z0-9 , SPACE and _	^[\w\s]+$
Only numeric 0-9	^[\d]+$  OR  ^[0-9]+$ OR [\d]+
Only numeric + dot ( amounts)	^[\d.]+$ OR ^[0-9.]+$ OR  [\d.]+
String of Length exactly 8	^[\w]{8}$
A string of min length 1 and max length 8	^[\w]{1,8}$
A number of fixed format 3.2 (e.g. 111.22)	^[\d]{3}\.[\d]{2}$
A number of min 1.0 and max 111.22 (e.g. 1 , 1.2, 22,22,444.22, but not 4444.44, 55.555)	^[\d]{1,3}\.[\d]{0,2}$

NOTES :
[1]
^ signifies start of string
$ signifies end of string
[] signifies one character
+ multiple characters

e.g. : ^[a-z]+$ : from start to end, for multiple characters ( no length restriction), verify that each character is in a-z.

[2]
\d : numeric
\s : white spaces
\w : numeric,character ( a-zA-Z) , underscore

\d\s : number + space
\w\s : alphanum + white space + _

[3]
"." matches any single character, but when included in a class [], it looses it special meaning and becomes charater dot "."

[4]

Note that . has to be escaped "\."

^[\d]{3}\.[\d]{2}$

In the above string, "." will be compulsory.

To make the "." optional, follow it with {0,1} :

^[\d]{1,3}\.{0,1}[\d]{0,2}$