Find a pattern to match 'a', ignoring that 'a' which lies within 'b' and 'c'

Question

Need a compound expression for

\" from\" such that \" from\" is not within parenthesis

(ignoring those which are in parenthesi

Answer 1

The following should work, even with arbitrarily nested parentheses:

if (Regex.IsMatch(subjectString, 
    @"\sfrom           # Match ' from'
    (?=                # only if the following regex can be matched here:
     (?:               # The following group, consisting of
      [^()]*           # any number of characters except parentheses,
      \(               # followed by an opening (
      (?>              # Now match...
       [^()]+          #  one or more characters except parentheses
      |                # or
       \( (?<DEPTH>)   #  a (, increasing the depth counter
      |                # or
       \) (?<-DEPTH>)  #  a ), decreasing the depth counter
      )*               # any number of times
      (?(DEPTH)(?!))   # until the depth counter is zero again,
      \)               # then match the closing )
     )*                # Repeat this any number of times.
     [^()]*            # Then match any number of characters except ()
     \z                # until the end of the string.
    )                  # End of lookahead.", 
    RegexOptions.IgnorePatternWhitespace))

As a single line regex ("The horror! The horror!"), if you insist:

if (Regex.IsMatch(subjectString,@"\sfrom(?=(?:[^()]*\((?>[^()]+|\((?<DEPTH>)|\)(?<-DEPTH>))*(?(DEPTH)(?!))\))*[^()]*\z)"))

Answer 2

This may be what you want.

string s="select field1 dfd t1 (select field1 from t1)select field1 from t1";
Regex r=new Regex(@"(?<=\)|^)\bselect\b.*?\bfrom\b.*?(?=\()",RegexOptions.RightToLeft);
r.Replace(s,"HELL yeah");