Replace every occurrence of "ab" with "X":

Replace every substring that has one or more occurrences of "ab" with "X":

Use the operator form of StringReplace:

Use string patterns:

Use a regular expression:

Use pattern matching for dates:

Mixed regular expressions and string patterns:

Use RuleDelayed (:>) in order to evaluate the right-hand side of a rule:

Replace only the two first occurrences that match the given pattern:

Replace occurrences of either string:

StringReplace works on special characters:

StringReplace automatically threads over lists of strings:

Change the nucleotides in a biomolecular sequence:

Use a wildcard in the pattern replaced in a given biomolecular sequence:

The "N" is a degenerate letter and is not a wildcard except in biomolecular sequences:

Additional wraparound replacements may be made in circular biomolecular sequences:

Replace only literal degenerate letter occurrences using Verbatim:

Replacements can be nonstrings:

Remove leading and trailing spaces:

Remove comments in a text:

Extract comments from a text:

Strip off all tags "<...>" from an HTML document:

Capitalize all words in a sentence:

Change the date convention in a text:

Identify prime numbers in a text:

A neighbor-independent substitution system:

Five steps in the evolution:

StringPosition gives the starting and ending positions at which a pattern occurs:

The output can be used with StringReplacePart:

Same result: