Parsing

Every infobox that hides an empty row, every template that prints a different label depending on a parameter, every wiki page that computes an age from a date relies on the same extension: ParserFunctions. It adds logic to wikitext, which on its own can only substitute text. This guide covers the functions you will actually use, and the three behaviours that explain most of the questions people ask about them. Everything below was checked against the extension's documentation on mediawiki.org on 15 October 2026.

Installation and syntax

ParserFunctions ships with MediaWiki. It has been bundled with the MediaWiki tarball since version 1.18, so on a self-hosted wiki you usually only need to enable it with `wfLoadExtension( 'ParserFunctions' );` in `LocalSettings.php`. It needs no database update. On a hosted wiki you cannot change this file, which matters for the configuration options below.

Every function has the same shape. You write `{{#name: first | second | third }}`: a hash, the function name, a colon, then parameters separated by pipes. The documentation states that whitespace, including newlines, is stripped from the beginning and end of every parameter. That is convenient for layout, and it is the source of the first trap.

The conditional functions

`#if` tests whether a string is empty, nothing else. `{{#if: test | value if not empty | value if empty }}` returns the second parameter when the first contains anything, and the third when it is empty or only contains whitespace. It does not evaluate the test: `{{#if: 1==2 | yes | no }}` returns yes, because "1==2" is a non-empty string, and `{{#if: 0 | yes | no }}` also returns yes. For arithmetic conditions, use `#ifexpr`.

• #if : non-empty string or not (whitespace counts as empty)
• #ifeq : two values identical or not (numeric if both are numbers)
• #iferror : did another function return an error
• #ifexpr : true or false result of an arithmetic expression
• #ifexist : does a page exist (expensive, limited per page)
• #switch : one result among several cases, with a default
• #expr : arithmetic, rounding, logic
• #time / #timel : format a date (UTC or local)
• #titleparts : split a page title on its slashes
• #rel2abs : turn a relative page path into an absolute one

`#ifeq` compares two strings, and numbers as numbers. `{{#ifeq: a | b | same | different }}` returns the third or fourth parameter. When both values are valid numbers, the comparison is numeric, so `01` equals `1` and `1e3` equals `1000`. If you need to compare more than two values, `#switch` is the better tool.

`#switch` picks one result among many. `{{#switch: value | case1 = result1 | case2 = result2 | default }}` returns the result of the first matching case, and the last parameter without an equals sign when nothing matches. Several cases can share one result by listing them one after another, with the result only on the last. It replaces a chain of nested `#ifeq` that becomes unreadable after three levels.

Arithmetic, dates and page checks

`#expr` does arithmetic, not string work. `{{#expr: 1 + 1 }}` returns 2, and the function supports logic operators, `round`, `trunc`, `ceil` and `floor`. An invalid expression returns an error message that `#iferror` can catch. The documentation is explicit that expressions only work with number-like values and cannot compare strings; use `#ifeq` for that. For dates, `#time` formats a date, and the documentation suggests the `xNU` format to turn it into seconds since 1 January 1970, which you can then add and subtract with `#expr`.

`#ifexist` is an expensive function, and it fails silently past a limit. It checks whether a page exists. Each distinct target page it checks counts against `$wgExpensiveParserFunctionLimit`, including checks made inside transcluded templates. Once the limit is reached, every further `#ifexist` returns false whether the page exists or not, and the page is added to a tracking category. If a template suddenly shows red links for pages that exist, this is the first thing to check.

The two traps that break templates

Raw pipes break tables. Because every pipe is read as a parameter separator, a wikitable written inside `#if` or `#switch` falls apart. Since MediaWiki 1.24, the `{{!}}` magic word outputs a pipe that the parser only sees after the function has run; before that, most wikis used a template named `!` for the same purpose. To display a literal pipe, use `|`.

The string functions are off by default. `#len`, `#pos`, `#sub`, `#replace`, `#explode` and their siblings only exist when `$wgPFEnableStringFunctions` is set to true, and its default is false; `$wgPFStringLengthLimit` caps the strings they accept at 1,000 characters by default. The documentation adds that in 2013 it was decided they will never be enabled on any Wikimedia wiki, because they are inefficient at scale. On Wikipedia and its sister projects, string handling goes through Lua modules instead, such as `{{#invoke:String|len|...}}` provided by Scribunto.