Autoincrement SQL server 2008/c# - c#-4.0

Question

How do I increment field of table using varchar. Here an example of what I want to have:
Mark 00001
Mark 00002
Mark 00003
Jaques 00001
Jaques 00002
Jaques 00003
Here is my example It can be useful for other people thanks to those who have helped me a lot thank you
The table is
`CREATE TABLE [dbo].[TAG_Sinistre](
[ID] [int] IDENTITY(1,1) NOT NULL,
[ref_ag] [varchar](7) NULL,
[ref_sinistre] [varchar](7) NULL,
)`
The stored procedure is
create PROC [dbo].[sp_Add_AgSinistre]
#ref_ag varchar (7)
AS BEGIN
declare #id int
DECLARE #ref_sin VARCHAR
SET #id = (SELECT ISNULL(MAX(CAST(ID AS INT)), 0) + 1
FROM TAG_Sinistre where ref_ag=#ref_ag
)
select #ref_sin=right('000000'+CAST(#ref_sin AS VARCHAR(6)),6)
BEGIN
INSERT into TAG_Sinistre(ref_ag,ref_sinistre)
VALUES (#ref_ag,#ref_sin)

Answer 1

Assuming the CompanyName field is UNIQUE, you could get the MAX added value, and increment that?
DECLARE #CurrentSequence INT
SET #CurrentSequence = (SELECT MAX(SequenceId)
FROM TableName WHERE CompanyName = #CompanyName)
INSERT INTO TableName
(CompanyName, SequenceId)
VALUES
(#CompanyName, #CurrentSequence+1)
Your StoredProc would pass in the Company Name as #CompanyName. Obviously, this is a naive approach, and I'm not including what would happen if multiple attempts to update the company would happen simultaneously, but there's no mention of that ion the question.
EDIT: Based on comments
DECLARE #maxSeq INT
SET #maxSeq = (SELECT ISNULL(MAX(CAST(SequenceId AS INT)), 0) + 1 FROM
TableName WHERE CompanyName = #CompanyName)
INSERT INTO TableName
(CompanyName, SequenceId)
VALUES
(#CompanyName, right('000000'+CAST(#maxSeq AS VARCHAR(7)),7))
As I said, I would look to use an INT, and get the UI to present the sequence as you want, instead of casting etc in the DB.
EDIT 2: Added Fiddle
SQL Fiddle

Answer 2

Assuming that your columns always look like those specified above.
DB Trigger (MS-SQL) --> Pseudocode/Untested:
CREATE TRIGGER [dbo].[YourTrigger]
ON [dbo].[YourTable]
INSTEAD OF INSERT
AS
BEGIN
DECLARE #maxVal int
-- PSEUDOCODE!!!
SELECT #maxVal = MAX(CAST(dbo.RegexReplace( '.*?(\d+)', myColumn, '$1') AS INT ))
FROM YourTable
INSERT INTO YourTable
SELECT myColumn + ' ' + #maxVal FROM inserted
END
Use insert now like this:
INSERT INTO YourTable values('Jaques')