Regex pattern for checking if a string starts with a certain substring

Question

What s the regular expression to check if a string starts with  mailto  or  ftp  or  joe  or     Now I am using C  and code like this in a big if with many ors   String StartsWith  mailto    String StartsWith  ftp     It looks like a regex would be better for this  Or is there a C  way I am missing here

User · Answer

For the extension method fans   public static bool RegexStartsWith this string str  params string   patterns        return patterns Any pattern   gt          Regex Match str       pattern      Success       Usage   var answer   str RegexStartsWith  mailto   ftp   joe      or var answer2   str RegexStartsWith  mailto ftp joe      or bool startsWithWhiteSpace      does this start with space or tab   RegexStartsWith    s

User · Answer

I really recommend using the String StartsWith method over the Regex IsMatch if you only plan to check the beginning of a string    Firstly  the regular expression in C  is a language in a language with does not help understanding and code maintenance  Regular expression is a kind of DSL  Secondly  many developers does not understand regular expressions  it is something which is not understandable for many humans  Thirdly  the StartsWith method brings you features to enable culture dependant comparison which regular expressions are not aware of    In your case you should use regular expressions only if you plan implementing more complex string comparison in the future

User · Answer

The following will match on any string that starts with mailto  ftp or http    RegEx reg   new RegEx    mailto ftp http       To break it down       matches start of line  mailto ftp http  matches any of the items separated by a     I would find StartsWith to be more readable in this case

User · Answer

The StartsWith method will be faster  as there is no overhead of interpreting a regular expression  but here is how you do it  if  Regex IsMatch theString   quot   mailto ftp joe   quot         The   mathes the start of the string  You can put any protocols between the parentheses separated by   characters  edit  Another approach that is much faster  is to get the start of the string and use in a switch  The switch sets up a hash table with the strings  so it s faster than comparing all the strings  int index   theString IndexOf       if  index    -1      switch  theString Substring 0  index         case  quot mailto quot       case  quot ftp quot       case  quot joe quot            do something       break

User · Answer

You could use     mailto ftp joe    But to be honest  StartsWith is perfectly fine to here  You could rewrite it as follows   string   prefixes      http    mailto    joe     string s    joe bloggs   bool result   prefixes Any prefix   gt  s StartsWith prefix      You could also look at the System Uri class if you are parsing URIs

[c#] Regex pattern for checking if a string starts with a certain substring?

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