Regular Expression

Special Symbol
 \d is digit
\s is white space
. any char
\w equal [A-Za-z0-9_]


boundary
^ is start of string
$ is end of string
\b word boundary

how many times (for previous item)
?  0 or 1
* 0 or more
+ 1 or more
{n} n times
{n,} at least n times
{n,m} n<=match times<=m

group
(pattern) group as one item
(x|y) x or y
[abc] either a,b or c
[^abc] any char except a,b,c
[a-d] match a,b.c or d

PHP related function
preg_match("/mypatter/i", $subject) return 1 if match; otherwise, return 0.  i is a flag for case insensitive search.

Example

all combinations of digits chars and spaces  /^([a-zA-Z\d\s]+)+$/  For this regular expression, empty string will fail