Regex to check if exact string exists including #

Question

New question As suggested by Asaph in previous question: Regex to check if exact string exists

I am looking for a way to check if an exact string match exists in another string using Regex or any better method suggested. I understand that you tell regex to match a space or any other non-word character at the beginning or end of a string. However, I don't know exactly how to set it up.

Search String: #t

Should Match:
String 1: Hello World, Nice to see you! #t
String 2: #T Hello World, Nice to see you!
String 3: Hello World, #t Nice to see you!

Should not Match:
String 1: Hello World, Nice to see you!
String 2: Hello World, Nice to see you! #ta
String 3: #tHello World, Nice to see you!

Edit 2: Added more string samples

Edit 1 for Serg555 and SilentGhost:
Characters allowed in search string:
#[_a-zA-Z0-9]
# is optional.

Requirements: Search String may be at any character position in the Subject. There may or may not be a white-space character before or after it. I do not want it to match if it is part of another string; such as part of a word.

For the sake of this question: I think I would do this using this pattern: /\b\#t\b/gi
However, this is not returning the results as I would have expected.

I am able to find the exact matches for normal strings (strings where # isn't present) using:

/\b{$search_string}\b/gi

Additional info: this will be used in PHP 5

Answer 1

All you need is:

/(?:^|\s)#t\b/i           #t is in the beginning or preceded by space.

\b matches word border, which is a border between word-characters and non-word characters. # is a non-word character, therefore your regex matches only strings like this: abc#t, or ab_#t.

Also, normally # is not a special character in regex, you don't need to escape it.

ETA: Your requirements are rather ambiguous: There may or may not be a white-space character before or after it. I do not want it to match if it is part of another string; such as part of a word.

1. no white space character before or after? so there will be a non white-space character?
2. but how is it separated then from other strings? what characters are allowed?

I think you need to give a comprehensive sample of your possible input strings. Because, as it is my regex works just fine.

Answer 2

Try /(?!\b)#t\b/si