SQL Server 2017

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DECLARE @initialString   VARCHAR(32) = '1234567890123456789'
DECLARE @startIdentifier VARCHAR(32) = '34'
DECLARE @endIdentifier   VARCHAR(32) = '34'
DECLARE @newString       VARCHAR(32) = 'ABC'

DECLARE @headChars INT = CHARINDEX(@startIdentifier, @initialString, 1)

IF @headChars > 0
  SET @headChars = @headChars + LEN(@startIdentifier) - 1

DECLARE @bodyChars INT = CHARINDEX(@endIdentifier, @initialString, @headChars + 1)

IF @bodyChars > 0
  SET @bodyChars = LEN(@initialString) - @bodyChars + 1

SELECT
  LEFT(@initialString,  @headChars)
  + @newString
  + RIGHT(@initialString, @bodyChars)

(No column name)

1234ABC3456789

DECLARE @initialString   VARCHAR(32) = '1234567890123456789'
DECLARE @startIdentifier VARCHAR(32) = '34'
DECLARE @endIdentifier   VARCHAR(32) = 'ZZ'
DECLARE @newString       VARCHAR(32) = 'ABC'

DECLARE @headChars INT = CHARINDEX(@startIdentifier, @initialString, 1)

IF @headChars > 0
  SET @headChars = @headChars + LEN(@startIdentifier) - 1

DECLARE @bodyChars INT = CHARINDEX(@endIdentifier, @initialString, @headChars + 1)

IF @bodyChars > 0
  SET @bodyChars = LEN(@initialString) - @bodyChars + 1

SELECT
  LEFT(@initialString,  @headChars)
  + @newString
  + RIGHT(@initialString, @bodyChars)

(No column name)

1234ABC

DECLARE @initialString   VARCHAR(32) = '1234567890123456789'
DECLARE @startIdentifier VARCHAR(32) = 'ZZ'
DECLARE @endIdentifier   VARCHAR(32) = '34'
DECLARE @newString       VARCHAR(32) = 'ABC'

DECLARE @headChars INT = CHARINDEX(@startIdentifier, @initialString, 1)

IF @headChars > 0
  SET @headChars = @headChars + LEN(@startIdentifier) - 1

DECLARE @bodyChars INT = CHARINDEX(@endIdentifier, @initialString, @headChars + 1)

IF @bodyChars > 0
  SET @bodyChars = LEN(@initialString) - @bodyChars + 1

SELECT
  LEFT(@initialString,  @headChars)
  + @newString
  + RIGHT(@initialString, @bodyChars)

(No column name)

ABC34567890123456789