Editing Sed (section)

===Substitution command===
The following example shows a typical, and the most common, use of sed: substitution. This usage was indeed the original motivation for sed:<ref name=early_history />
<syntaxhighlight lang=bash>
sed 's/regexp/replacement/g' inputFileName > outputFileName
</syntaxhighlight>
In some versions of sed, the expression must be preceded by <code>-e</code> to indicate that an expression follows. The <code>s</code> stands for substitute, while the <code>g</code> stands for global, which means that all matching occurrences in the line would be replaced. The [[regular expression]] (i.e. pattern) to be searched is placed after the first delimiting symbol (slash here) and the replacement follows the second symbol. Slash (<code>/</code>) is the conventional symbol, originating in the character for "search" in ed, but any other could be used to make syntax more readable if it does not occur in the pattern or replacement; this is useful to avoid "[[leaning toothpick syndrome]]".

The substitution command, which originates in search-and-replace in ed, implements simple parsing and [[templating language|templating]]. The <code>regexp</code> provides both pattern matching and saving text via sub-expressions, while the <code>replacement</code> can be either literal text, or a format string containing the characters <code>&amp;</code> for "entire match" or the special [[escape sequence]]s <code>\1</code> through <code>\9</code> for the ''n''th saved sub-expression. For example, <code>sed -r "s/(cat|dog)s?/\1s/g"</code> replaces all occurrences of "cat" or "dog" with "cats" or "dogs", without duplicating an existing "s": <code>(cat|dog)</code> is the 1st (and only) saved sub-expression in the regexp, and <code>\1</code> in the format string substitutes this into the output.