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regexp-make-js

RegExp.make is an ES6 string template tag for dynamically creating regular expressions.

Usage

RegExp.make`^${foo},${bar}$`

is a RegExp instance that matches the whole string (^...$) consisting of a substring matching the value of the expression foo followed by the literal substring "," followed by a substring matching the value of the expression bar.

Interpolated expressions like foo and bar can be strings, or RegExp instances, or other values that are coerced to strings.

RegExp instances are treated like the set of substrings they match -- their source is not used as a literal string.

RegExp.make`^${ /fo+/ }$`

matches the entire string consisting of 'f' followed by one or more 'o's; the Kleene + is not treated literally.

Goals

This currently uses the subset of EcmaScript 2015 (ES6) that is implemented on FF >= 39. To see the test visit the test page in your browser using Firefox.

This is a proposed alternative to RegExp.escape. To get simply the equivalent functionality of RegExp.escape, anywhere you would have said

RegExp.escape(str)

you can say instead

RegExp.make`${str}`.source

However, if you do only that you have not gained anything. The advantage of using the tag is that it can do reliable context-dependent escaping of the string as interpolated into RegExp source text. Where you might have said, for example,

const re = new RegExp('^(' + RegExp.escape(str) + ')$');

with RegExp.make you can say instead

const re = RegExp.make`^(${str})$`;

Expressions

Context Example String Numeric RegExp
Block /${...}/ Treated literally Treated Literally With back-references adjusted
Charset /[^${...}]/ Individual chars Individual Chars All chars in any string matched by the RegExp
Count /x{1,${...}}/ Inlined without wrapping Inlined without wrapping Inlined without wrapping

Interpolated values are treated as atoms so

RegExp.make`${foo}*`

matches any number of the pattern specified by foo; it's not just the last character in that pattern that the Kleene star applies to.

Flags

RegExp.make('i')`^${foo}$`

applies the i flag (case-insensitive) to the RegExp after interpolation happens, so substrings matched by the expression foo are matched case-insensitively.

When a case-insensitive RegExp is interpolated into a case-sensitive one, the interpolated one still matches case insensitively.

RegExp.make`foo-${ /bar/i }`

matches "foo-BAR" but not "FOO-BAR".

Groups

RegExps produced have the templateGroups property set so that if values specify groups, you can figure out the group index of a group specified by the template.

var re = RegExp.make`${ /(foo)/ }(\d+)`;
//           value group ^       ^ template group 1
var match = "foo123".match();
match[1] === 'foo';  // Because of /(foo)/
match[re.templateGroups[1]] === '123';

TODO

  • The u flag is not recognized and it should affect how we do case-folding and treat ., \w character classes, \u{...} escapes, etc.