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 allow only unique numbers in a given range, you can use data validation with a custom formula based on the AND, ISNUMBER, and COUNTIF functions.
In the example shown, the data validation applied to B5:B9 is:
=AND(...

Note: Excel contains many built-in "presets" for highlighting values with conditional formatting, including a preset to highlight cells that contain specific text. However, if you want more flexibility, you can use your...

To extract all matches based on a partial match, you can use use an array formula based on the INDEX and AGGREGATE functions, with support from ISNUMBER and SEARCH. In the example shown, the formula in G5 is:
=IF(F5...

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

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 count the number of specific words or keywords that appear in a given cell, you can use a formula based on the SEARCH, ISNUMBER, and SUMPRODUCT functions. In the example shown, the formula in C5 is:
=SUMPRODUCT(--...

If you want to test a cell to see if it contains all items in a list, you can do so with a formula that uses the SEARCH function, with help from the ISNUMBER, SUMPRODUCT, and COUNTA functions.
Context
Let's say you...

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...

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...

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 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 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.
This is an array formula and must be entered...

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 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 "*"...