Supported Modifier Flags · Issue #1 · tc39/proposal-regexp-modifiers (original) (raw)
Navigation Menu
- Explore
- Pricing
Provide feedback
Saved searches
Use saved searches to filter your results more quickly
Appearance settings
Description
In the Oct, 2021 plenary, @michaelficarra asked that we outline and provide motivating examples for each flag we are considering as a supported modifier.
The flags currently under consideration are:
i— ignore-case- Rationale — Toggling ignore-case is especially useful when matching patterns with varying case sensitivity, or when parsing patterns provided via JSON configuration. Especially useful when working with complex Unicode character ranges.
- Example — Match upper case ascii letter followed by upper or lower case ascii letter or '
const re = /^A-Z[a-z']+$/;
re.test("O'Neill"); // true
re.test("o'neill"); // false
// alternatively (defaulting to ignore-case):
const re2 = /^(?-i:[A-Z])[a-z']+$/i; - Example — Match word starting with
Dfollowed by word starting withDord(from .NET documentation, see 1)
const re = /\b(D\w+)(?ix)\s(d\w+)\b/g;
const input = "double dare double Double a Drooling dog The Dreaded Deep";
re.exec(input); // ["Drooling dog", "Drooling", "dog"]
re.exec(input); // ["Dreaded Deep", "Dreaded", "Deep"]
m— multiline- Rationale — Flexibility in matching beginning-of-buffer vs. beginning-of-line or end-of-buffer vs. end-of-line in a complex pattern.
- Example — Match a frontmatter block at the start of a file
const re = /^---(?m)$((?:^(?!---$).$))^---$/;
re.test("---a"); // false
re.test("---\n---"); // true
re.test("---\na: b\n---"); // true
s— dot-all (i.e., "single line")- Rationale — Control over
.matching semantics within a pattern. - Example
const re = /a.c(?s:.)*x.z/;
re.test("a\ncx\nz"); // flse
re.test("abcdxyz"); // true
re.test("aBc\nxYz"); // true
- Rationale — Control over
x— Extended Mode. This flag is proposed by https://github.com/tc39/proposal-regexp-x-modeRationale — Would allow control over significant whitespace handling in a pattern.
Example — Disabling
xmode when composing a complex pattern:
const idPattern =[a-z]{2} \d{4}; // space required
const re = new RegExp(String.raw`match the id
(?(?-x:${idPattern}))
match a separator
:\s
match the value
(?\w+)
`, "x");
re.exec("aa0123: foo")?.groups; // undefined
re.exec("aa 0123: foo")?.groups; // { id: "aa 0123", value: "foo" }
Flags likely too complex to support:
u— Unicode. This flag affects how a pattern is parsed, not how it is matched. Supporting it would likely require a cover grammar and additional static semantics.v— Extended Unicode. This flag is proposed by https://github.com/tc39/proposal-regexp-set-notation as an extension of theuflag and would have the same difficulties.
Flags that will never be supported:
g— Global. This flag affects the index at which matching starts and not the matching behavior itself. Changing it mid pattern would have no effect.y— Sticky. This flag affects the index at which matching starts and not the matching behavior itself. Changing it mid pattern would have no effect.d— Indices. This flag affects the match result. Changing it mid pattern would have no effect.