I need to stop doing this to myself, solved it.
The code is:
=SUBSTITUTE(A1, "_", "_mobile_", 1)
It will substitute any "_" it finds, but only the FIRST _ found in each text string located in A1. The replacement word is "_mobile_".
That is not "code"... it's a formula and, as such, must be placed in a different column from the data. However, when you said...
"I'm trying to figure out how to add the word mobile to a line of cells - A column"
in your original message, it sounded like you wanted to physically change the original data values within the cells of Column A themselves, not show the changed value in another cell. If that is what you really wanted, you would need a macro, like this one (remember, if you run it, it will physically change the values in Column A directly)...
Code:
Sub PutWordAfterFirstUnderline()
Dim Addr As String
Const Word As String = "mobile"
Addr = "A1:A" & Cells(Rows.Count, "A").End(xlUp).Row
Range(Addr) = Evaluate("IF(LEN(" & Addr & "),SUBSTITUTE(" & Addr & ",""_"",""_" & Word & "_"",1),"""")")
End Sub
HOW TO INSTALL MACROs
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If you are new to macros, they are easy to install and use. To install it, simply press ALT+F11 to go into the VB editor and, once there, click Insert/Module on its menu bar, then copy/paste the above code into the code window that just opened up. That's it.... you are done. To use the macro, go back to the worksheet with your data on it and press ALT+F8, select the macro name (PutWordAfterFirstUnderline) from the list that appears and click the Run button. The macro will execute and perform the action(s) you asked for. If you are using XL2007 or above, and you want to retain this macro for use in the future, make sure you save your file as an "Excel Macro-Enabled Workbook (*.xlsm) and answer the "do you want to enable macros" question as "yes" or "OK" (depending on the button label for your version of Excel) the next time you open your workbook.