Note: Excel has several built-in data validation rules for numbers. This page explains how to create a your own validation rule based on a custom formula.
To allow only numbers in a cell, you can use data validation...

To test if one of several values exists in a range of cells, you can use a formula based on the SUMPRODUCT function. In the example, the formula in cell D5 is:
=SUMPRODUCT(--(rng=B5:D5))>0
How the formula works...

To test a cell for one of several things, and return a custom result for the first match found, you can use an INDEX / MATCH formula based on the SEARCH function.
In the example shown, the formula in C5 is:
{=INDEX(...

To count cells that do not contain many different strings, you can use a rather complex formula based on the MMULT function. In the example shown, the formula in F5 is:
{=SUM(1-(MMULT(--(ISNUMBER(SEARCH(TRANSPOSE(...

To list the most frequently occurring numbers in a column (i.e. most common, second most common, third most common, etc), you can an array formula based on four Excel functions, IF, MODE, MATCH, and ISNUMBER. In the...

To get the position of the first match that does not contain a specific value, you can use an array formula based on the MATCH, SEARCH, and ISNUMBER functions. In the example shown, the formula in E5 is:
{=MATCH(FALSE...

To test a cell for one of many strings, while excluding others, you can use a formula based on the SEARCH, ISNUMBER, and SUMPRODUCT functions.
In the example shown the formula in C5 is:
=(SUMPRODUCT(--ISNUMBER(SEARCH...

To allow only values that contain a specific text string, you can use data validation with a custom formula based on the FIND and ISNUMBER functions. In the example shown, the data validation applied to C5:C9 is:
=...

To categorize text using keywords with a "contains" match, you can use the SEARCH function, with help from INDEX and MATCH. In the example shown, the formula in C5 is:
{=INDEX(categories,MATCH(TRUE,ISNUMBER(SEARCH(...

To count the number of cells that contain certain text, you can use the COUNTIF function. In the generic form of the formula (above), rng is a range of cells, txt represents the text that cells should contain, and "*"...

If you want to test a cell to see if it contains one of several things, you can do so with a formula that uses the SEARCH function, with help from the ISNUMBER and SUMPRODUCT functions.
Context
Let's say you have a...

To highlight cells that contain one of many text strings, you can use a formula based on the functions ISNUMBER and SEARCH, together with the SUMPRODUCT function. In the example shown, the conditional formatting applied...

If you have a list of things (words, substrings, etc) and want to find out which of these things appear in a cell, you can build a simple table and use a formula based on the SEARCH function.
Setup
Suppose you have a...

To highlight numbers less than a certain value, including numbers entered as text like "

To test a range for numbers, you can use a formula based on the ISNUMBER and SUMPRODUCT functions.
In the example shown, the formula in xxxx is:
=SUMPRODUCT(--ISNUMBER(C5:C9))>0
How this formula works
Working...