Description:
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This phrase in the documentation is a bit contradictory:
"preg_match() returns the number of times pattern matches. That will be either 0 
times (no match) or 1 time because preg_match() will stop searching after the 
first match."

The first part says that preg_match returns the number of times a patterne 
matches, but the second part says that it only returns 0 or 1... I suggest to 
change it with something like:

preg_match() returns either 0 (no match) or 1(at least one match is found), 
because it will stop searching after the first match.