Here's one way to do it in T-SQL:

DECLARE @String VARCHAR(8000) = 'ABC12D34E56';

WITH Seeds(Seed)

AS (SELECT *

FROM ( VALUES ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1) ) AS V (C)),

Numbers(Number)

AS (SELECT TOP (8000)

ROW_NUMBER() OVER (ORDER BY S1.Seed)

FROM Seeds AS S1

CROSS JOIN Seeds AS S2

CROSS JOIN Seeds AS S3

CROSS JOIN Seeds AS S4)

SELECT (

SELECT SUBSTRING(@String, Number, 1)

FROM Numbers

WHERE Number <= LEN(@String)

AND SUBSTRING(@String, Number, 1) LIKE '[0-9]'

ORDER BY Number

FOR XML PATH(''),

TYPE).value('.[1]', 'VARCHAR(8000)');

Alternatively, if you need to apply this to a table instead of a variable:

IF OBJECT_ID(N'tempdb..#T') IS NOT NULL

DROP TABLE #T;

CREATE TABLE #T

(ID INT IDENTITY

PRIMARY KEY,

Col1 VARCHAR(8000));

INSERT INTO #T

(Col1)

VALUES ('123A'),

('B1C2D3');

WITH Seeds(Seed)

AS (SELECT *

FROM ( VALUES ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1), ( 1) ) AS V (C)),

Numbers(Number)

AS (SELECT TOP (8000)

ROW_NUMBER() OVER (ORDER BY S1.Seed)

FROM Seeds AS S1

CROSS JOIN Seeds AS S2

CROSS JOIN Seeds AS S3

CROSS JOIN Seeds AS S4)

SELECT *

FROM #T AS T

CROSS APPLY (SELECT (

SELECT SUBSTRING(Col1, Number, 1)

FROM Numbers

WHERE Number <= LEN(Col1)

AND SUBSTRING(Col1, Number, 1) LIKE '[0-9]'

ORDER BY Number

FOR XML PATH(''),

TYPE).value('.[1]', 'VARCHAR(8000)') AS Stripped) AS Parser;

In either case, it uses a "Runtime Numbers Table" to parse the string into individual characters, then strips out anything other than digits 0-9, then uses For XML to put it all back together. (FOR XML PATH with a zero-length root indicated by (''), and no column name for the query, will concatenate strings together nicely. It's a documented trick that comes in very handy.)

Does that help?