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Returns a modified copy of a given text, with the first occurrence of a find-string replaced with a replacement-string.
Equal signs in parameter values passed as immediate text either need to be escaped as ((=)), or you can leave them unescaped and use the built-in numeric names (|1=, |2=, and |3=) for the three unnamed positional parameters, as in the section usage at the top.
Pipe characters in immediate text should be escaped either as | or as ((!)). See Help:Template#Usage hints and workarounds. Pipe characters in embedded items such as params or template invocations don't need to be escaped.
Pay attention to the use of white space in the parameters; all white space is significant in positional parameters. In particular, leading and trailing white space is not removed, and can affect the results. See contrasting examples #1 and #2 below.
If you like the readability of having convenience blanks before and after the parameters so they're not adjacent to the pipe characters, then use the built-in numeric names |1=, etc. These will not include any leading or trailing blanks in the parameter value; see example 3.