More likely: s = "something < other" s = s.replace("<", "&lt;") Even more likely: import cgi s = cgi.escape(s) In Python, much string manipulation is done with

More likely: s = "something < other" s = s.replace("<", "&lt;") Even more likely: import cgi s = cgi.escape(s) In Python, much string manipulation is done with builtins (especially string methods) and library functions. The latter work especially well when you target a specific domain (such as HTML) and will often be implemented in regexps. Writing one's own regexps are for when you actually need a small custom state machine. Many problems are too small or too large to justify the effort.