php - Validate string against regex -

i'd validate file paths regular expression. far i've come this:

preg_match('/[^a-za-z0-9\.\/\\\\]/', $string); 

so return 1 if string has other characters a-z, a-z, 0-9, dot, \ , /

how can make returns 1 if there more 2 dots in string, or if dot @ end of string?

and how can allow : if it's present 2nd character , followed \ or /. example c:\file should valid

for first 2 requirements:

preg_match('/[^a-za-z0-9\.\/\\\\]|\..*\.|\.$/', $string); 

the \..*\. match if there more 2 dots. \.$ match if there dot @ end. separating each of these portions (including original regex) | make regex match 1 of expressions (this called alternation).

the last requirement little tricky, since if understand correctly need return 1 if there :, unless colon second character , followed \ or /. following regex (as php string) should that:

'/:(?!(?<=^[a-za-z]:)[\/\\\\])/' 

or combined other regex (note have add : first character class):

preg_match('/[^a-za-z0-9\.\/\\\\:]|\..*\.|\.$|:(?!(?<=^[a-za-z]:)[\/\\\\])/', $string); 

here explanation last piece:

:                  # match ':' (?!                # fail if following regex matches (negative lookahead)    (?<=^[a-za-z]:)   # ':' second character in string    [\/\\\\]          # next character '/' or '\' )                  # end lookahead