PHP Regular expression (pattern modifier)

Eighth chapter (3) Regular expression (pattern modifier)
Matching sequence of regular expressions:
1. Mode Unit 2. Duplicate match? * + {} 3. boundary limit ^ $ b b 4. Mode selection |
Pattern modifier:
The pattern modifier is marked outside the entire pattern.
I: Characters in the pattern will match both uppercase and lowercase letters.
M: string as multiple lines.
S: Treats a string as a single line, and a newline character as a normal.
x: Ignore whitespace in the pattern.
A: Coercion only starts at the beginning of the target string.
D: The dollar character in the pattern matches only the end of the destination string.
U: Matches the nearest string.