1. Regular Expressions
    1. Regular Expression Syntax
      1. Substitution
      2. Ambiguity

Regular Expressions

Regular expressions are available in the Find... and Replace... dialogs as a way to match inexact sequences of characters. Regular expression substitution can also be used to program automatic editing operations. For example, the following are search and replace strings to find occurrences of the subroutine get_x, reverse the first and second parameters, add a third parameter of NULL, and change the name to new_get_x":

Search string: get_x\(([^ ,]*), ([^\)]*)\)
Replace string: new_get_x(\2, \1, NULL)

To use regular expressions, click on the Regular Expression button in the Find... or Replace... dialogs before doing a search or replacement.

Regular Expression Syntax

The components of a regular expression are: branches, pieces, atoms, and ranges. A regular expression consists of zero or more branches, separated by `|'. It matches anything that matches one of the branches.

A branch is zero or more pieces, concatenated. It matches a match for the first, followed by a match for the second, etc.

A piece is an atom possibly followed by `*', `+', or `?'. An atom followed by `*' matches a sequence of 0 or more matches of the atom. An atom followed by `+' matches a sequence of 1 or more matches of the atom. An atom followed by `?' matches a match of the atom, or the null string.

An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), `.' (matching any single character), `^' (matching the null string at the beginning of a line string), `$' (matching the null string at the end of a line), `<' or `>' (matching the null string at a word boundary), a `\' followed by a single character (matching that character), or a single character with no other significance (matching that character). \t, \n, \b, \r, and \f represent the characters tab newline, backspace, carriage return, and form feed.

A range is a sequence of characters enclosed in `[]'. It normally matches any single character from the sequence. If the sequence begins with `^', it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by `-', this is shorthand for the full list of ASCII characters between them (e.g. `[0-9]' matches any decimal digit). To include a literal `]' in the sequence, make it the first character (following a possible `^'). To include a literal `-', make it the first or last character. A backslash `\' followed by a single character includes that character, however backslashes are not necessary for most special characters, since inside a range, only the `]', `-', and '\' characters are treated specially.


Wherever the substitution string contains the character `&', NEdit will substitute the entire string that was matched in the Find operation. Up to nine sub-expressions of the match string can also be inserted into the replacement string, using `\' followed by a digit. \1 through \9 represent the strings that matched parenthesized expressions within the regular expression, numbered left-to-right in order of their opening parentheses. Preceding & or \1-9 with \U, \u, \L, or \l adjusts the case of the inserted text. \u and \l change only the first character, while \U and \L change the entire string to upper or lower case. \t, \n, \b, \r, and \f represent the characters tab newline, backspace, carriage return, and form feed in a substitution string represent the tab and newline characters as they do in match strings.


If a regular expression could match two different parts of the text, it will match the one which begins earliest. If both begin in the same place but match different lengths, or match the same length in different ways, life gets messier, as follows.

In general, the possibilities in a list of branches are considered in left-to-right order, the possibilities for `*', `+', and `?' are considered longest-first, nested constructs are considered from the outermost in, and concatenated constructs are considered leftmost-first. The match that will be chosen is the one that uses the earliest possibility in the first choice that has to be made. If there is more than one choice, the next will be made in the same manner (earliest possibility) subject to the decision on the first choice. And so forth.

For example, `(ab|a)b*c' could match `abc' in one of two ways. The first choice is between `ab' and `a'; since `ab' is earlier, and does lead to a successful overall match, it is chosen. Since the `b' is already spoken for, the `b*' must match its last possibility-the empty string-since it must respect the earlier choice.

In the particular case where no `|'s are present and there is only one `*', `+', or `?', the net effect is that the longest possible match will be chosen. So `ab*', presented with `xabbbby', will match `abbbb'. Note that if `ab*' is tried against `xabyabbbz', it will match `ab' just after `x', due to the begins-earliest rule. (In effect, the decision on where to start the match is the first choice to be made, hence subsequent choices must respect it even if this leads them to less-preferred alternatives.)

Send questions and comments to: nedit_support@fnal.gov.