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

Note: these is an experimental formula that uses a hard coded array constant, set down here for reference and comment. Casually tested only, so take care if you use or adapt.
Sometimes you encounter data that mixes...

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 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...

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 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 shown, the formula in cell D5 is:
=SUMPRODUCT(--(rng=B5:D5))>0
where "rng" is the...

To disallow input that contains one of many things, you can use a custom data validation rule based the SEARCH function.
In the example shown, the data validation applied to B5:B11 is:
=SUMPRODUCT(--ISNUMBER(SEARCH(...

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(...

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 filter data to include data based on a "contains specific text" logic, you can use the FILTER function with help from the ISNUMBER function and SEARCH function. In the example shown, the formula in F5 is:
=FILTER(...

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 check a cell for one of several things, and return the first match found in the list, you can use an INDEX / MATCH formula that uses SEARCH or FIND to locate a match. In the example shown, the formula in C5 is:
{=...

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...

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 "*"...

To check if a cell contains specific text, you can use the SEARCH function together with the ISNUMBER function. In the generic version, substring is the specific text you are looking for, and text represents text in the...