This example is not a reference on how to write regular expressions (complex regular expression writing can be very difficult) but we will look at some common operators and how you might use them. The information provided here is only a very basic introduction to extended regular expressions.

An important thing to remember is that ~ is the operator that controls regular expression matching within Ignite-UX configuration files. When writing regular expressions there are several operators you might find useful67:

∙“^” is an anchor to the start of what is being matched against. This might allow you to say something like “^u” which would match anything that started with a “u”. Without the “^”, a regular expression that just contained “u” would match any occurrence of “u” in what was being matched against.

∙“$” is an anchor to the end of what is being matched against. This might allow you to say something like “12$” which would match anything that ended with “12”.

∙“” in a regular expression acts as an “or” operator. This allows you to give several regular expressions together where either expression is allowed to match. For example “^user^sysadm” would match anything that started with “user” or “sysadm”.

∙“.” allows you to say any character. For example, “u.s” would match anything that contained (no anchor is in the expression) the letter “u” followed by any character followed by the letter “s”.

∙“[]” forms a bracket expression in a regular expression. This construct can be used to give multiple values that can be used to match a single character. For example “[ahj]” means anything that contains the letters “a”, “h” or “j”.

∙You can give a character class in the regular expression, for example “[:alpha:]” can be used to match any letter (upper or lowercase). The manual page regexp(5) contains more information on character classes (including the names of other character classes.) Note that you must use this within a bracket expression68 so it will end up looking like

“[[:alpha:]]”.

∙“*” can be used to perform wildcard matching, the regular expression “u*r” matches anything containing a “u” followed by zero or more characters followed by an “r”. For example, this would match “ur”, “usr”, and “user” (and many other things.)

∙Do not use the construct “.*” (zero or more of any character) at the start or end of an unanchored regular expression, it is pointless. For example “.*user.*” means zero or more of any character followed by “user” followed by zero or more of any character. The reason why this is pointless is that “user” by itself will match any part of the string it is being compared against. If compared against the string “auser” it will match without any additional “.*” in the regular expression.

∙“+” Allows you to match one or more occurrences of a pattern within an expression.

Note that the meaning of any of the regular expression operators mentioned above can change depending on their position. An example of this would be “” inside a bracket expression: inside a bracket expression it means the character “” not the “or” operator. Please refer to the regexp(5) manual page for additional information.

Now we can look at the use of regular expressions for adding some sanity checks against the user name from Example 5:

67Some of these explanations have been simplified; you should consult the regexp(5) manual page for more complete explainations.

68Outside of a bracket expression [:alpha:] in a regular expression would mean match the characters “:alpha” but [[:alpha:]] means match any upper or lower case letter.

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