Đại cương về regular expression php

Question mark in regular expression (?i)
Modern regex flavors allow you to apply modifiers to only part of the regular expression. If you insert the modifier (?ism) in the middle of the regex, the modifier only applies to the part of the regex to the right of the modifier. You can turn off modes by preceding them with a minus sign. All modes after the minus sign will be turned off. E.g. (?i-sm) turns on case insensitivity, and turns off both single-line mode and multi-line mode.

Not all regex flavors support this. JavaScript and Python apply all mode modifiers to the entire regular expression. They don't support the (?-ismx) syntax, since turning off an option is pointless when mode modifiers apply to the whole regular expressions. All options are off by default.

You can quickly test how the regex flavor you're using handles mode modifiers. The regex (?i)te(?-i)st should match test and TEst, but not teST or TEST.

?i means that everything following these characters should be matched case-insensitive.
Looking Inside the Regex Engine
Let's see what happens when we apply the regex \bis\b to the string This island is beautiful. The engine starts with the first token \b at the first character T. Since this token is zero-length, the position before the character is inspected. \b matches here, because the T is a word character and the character before it is the void before the start of the string. The engine continues with the next token: the literal i. The engine does not advance to the next character in the string, because the previous regex token was zero-width. i does not match T, so the engine retries the first token at the next character position.

\b cannot match at the position between the T and the h. It cannot match between the h and the i either, and neither between the i and the s.

The next character in the string is a space. \b matches here because the space is not a word character, and the preceding character is. Again, the engine continues with the i which does not match with the space.

Advancing a character and restarting with the first regex token, \b matches between the space and the second i in the string. Continuing, the regex engine finds that i matches i and s matches s. Now, the engine tries to match the second \b at the position before the l. This fails because this position is between two word characters. The engine reverts to the start of the regex and advances one character to the s in island. Again, the \b fails to match and continues to do so until the second space is reached. It matches there, but matching the i fails.

But \b matches at the position before the third i in the string. The engine continues, and finds that i matches i and s matches s. The last token in the regex, \b, also matches at the position before the third space in the string because the space is not a word character, and the character before it is.

The engine has successfully matched the word is in our string, skipping the two earlier occurrences of the characters i and s. If we had used the regular expression is, it would have matched the is in This.