C# and regular expressions (regex) - part 2

Some people say there are two types, POSIX and Perl. This is by no means true, one page claims that there are 418 regex flavors.

• Regular Expressions
• Regex flavors

Quick regex sampler

The simplest case is just to match a word like "bill" or "console" or "return".

The next thing you can do is to add an anchor.
Putting a "^" at the start of a regex anchors it to the start of a line.
Putting a "$" at the end of a regex anchors it to the end of a line.

So the regex "^sam" will match "sam is here", but not "I see sam".
Likewise the regex "sam$" will match "I see sam", but not "sam is here".

The next useful thing is "." which matches any character.
So "f..d" will match both "food" and "ford", but "flood" or "fad".

And the next thing to know about is "*" which says to match any number of whatever character (or character "thing") comes before it.
So "f.*d" will match "food", "ford", "flood" and "fad".

The last thing I will mention in this sampler is character classes. You put stuff inside of square brackets and they match a single character.
So "^[Ss]" will match any line with a big or little "s" at the start.

The pattern "[abc]*" will match any number of a, b, or c letters.
So it will match "a" or "aaaaaaaaa" or "abc" or "aabbcc" or "abcabc" and so on.

You can use a range inside of the brackets.
So "^[0-9]*" will match any number of digits at the start of a string. We are just getting warmed up. When I use vim to do searches, I often anchor a word to the start of a line and search for "^calc" if I have a function named "calc". This skips all the calls to calc in the middle of lines and in general takes me to the function definition. Vim also can do regex subtitution over the whole file, or parts of it, which can be very handy. I can do things like put a comment mark on the first of 100 lines of code by something like:

100,200s/^/#  /

More about regex