^ and $ are line anchors.

 
^     specifies the beginning of the line.

$     specifies the end of the line.

1.	Brackets [] finds a range of characters.
2.	Character Classes
3.	Complete list of regular expression examples
4.	\b and \B, equate to "On a word boundary" and "Not on a word boundary," respectively.
5.	Line Anchors
6.	Match URL
7.	Match an IP address
8.	Match the smallest number of characters starting with "p" and ending with "t"
9.	Matching GUIDs/UUIDs
10.	Matching a Valid E-mail Address
11.	Matching a Valid IP Address
12.	Matching using backreferences
13.	Matching with Greedy vs. Nongreedy Expressions
14.	Matching with character classes and anchors
15.	Matching with \|
16.	Define a pattern and use parentheses to match individual elements within it
17.	Greedy Qualifiers
18.	Greedy and non-greedy matching
19.	Greedy versus nongreedy matching
20.	Grouping captured subpatterns
21.	Validating Pascal Case Names
22.	Validating U.S. Currency
23.	Validating a credit card number
24.	Nongreedy Qualifiers
25.	POSIX Regular Expressions Character Classes
26.	POSIX Regular Expressions Character Classes
27.	Ranges
28.	Option patterns:
29.	Predefined Character Ranges (Character Classes)
30.	Pattern matches:
31.	Pattern match extenders:
32.	Perl-Compatible Regular Expressions (PCRE)
33.	Qualifiers restrict the number of times the preceding expression may appear.
34.	Quantifiers for Matching a Recurring Character
35.	Quantifiers: +, *, ?, {int. range}, and $ follow a character sequence:
36.	Special classes for regular expression
37.	Regular expressions using character classes

^ and $ are line anchors. : Regular Expressions « String « PHP