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Epsilon User's Manual and Reference > Commands by Topic > Changing Text > Regular Expressions >

Regular Expression Assertions

And with old woes new wail my dear times's waste;

Even though the word "new" occurs before "waste", it does not appear at the beginning of the line, so Epsilon rejects it.

Other assertions use Epsilon's angle-bracket syntax. Like the assertions ^ and $, these don't match any specific characters, but a potential match will be rejected if the assertion isn't true at that point in the pattern.

	Assertion	Meaning
	^	At the start of a line.
	$	At the end of a line.
	<bob> or <bof>	At the start of the buffer.
	<eob> or <eof>	At the end of the buffer.

For example, searching for <bob>sometext<eob> won't succeed unless the buffer contains only the eight character string sometext.

You can create new assertions from character classes specified with the angle bracket syntax by adding [, ] or / at the start of the pattern.

	Assertion	Meaning
	<[class>	The next character matches class, the previous one does not.
	<]class>	The previous character matches class, the next one does not.
	</class>	Either of the above.

The class in the above syntax is a |-separated or &-separated list of one or more single characters, character names like Space or Tab, character numbers like #32 or #9, ranges of any of these, character class names like Word or Digit, or Unicode property specifications. See Character Classes for details on character classes.

For example, </word> matches at a word boundary, and <]word> matches at the end of a word. The pattern <]0-9|a-f> matches at the end of a run of hexadecimal digits. The pattern (cat|[0-9])</digit>(dog|[0-9]) matches cat3 or 4dog, but not catdog or 42. The pattern <[p:cyrillic> matches at the start of a run of Cyrillic characters.

Color Class Assertions

Another type of assertion matches based on the next character's color class for syntax highlighting. <^c:*comment>printf finds uses of printf that aren't commented out. <[c:perl-string>" finds " characters that start a string in Perl mode, ignoring those that end it, or appear quoted inside it, or in comments or other places.

The text after the c: is a simple filename-style pattern that will be matched against the name of the color class: * matches zero or more characters, ? matches any single character, and simple ranges with [] are allowed. A character with no syntax highlighting applied will match the name "none". This type of assertion may start with ^ to invert the matching rules, or with /, [ or ] to match color boundaries.

To apply more than one assertion to a character, put them in sequence. <^c:perl-string><^c:*comment>printf finds instances of printf that are in neither Perl strings nor comments.

You can use the set-color command to see the color class names Epsilon uses.

When you combine assertions with operators * or +, you must use parentheses to specify that the assertion applies to each character. (<^c:*-comment><any>)+ matches a run of non-comment characters. Without the parentheses the assertion only applies to the first character of the run.

In extension language code, use the do_color_searching( ) subroutine if your regular expression might include syntax highlighting assertions, which ensures the buffer's syntax highlighting is up to date.

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Epsilon Programmer's Editor 14.04 manual. Copyright (C) 1984, 2021 by Lugaru Software Ltd. All rights reserved.