72 lines
2.4 KiB
Markdown
72 lines
2.4 KiB
Markdown
# Alternation (OR) |
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Alternation is the term in regular expression that is actually a simple "OR".
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In a regular expression it is denoted with a vertial line character `pattern:|`.
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[cut]
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For instance, we need to find programming languages: HTML, PHP, Java or JavaScript.
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The corresponding regexp: `pattern:html|php|java(script)?`.
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A usage example:
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```js run
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let reg = /html|php|css|java(script)?/gi;
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let str = "First HTML appeared, then CSS, then JavaScript";
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alert( str.match(reg) ); // 'HTML', 'CSS', 'JavaScript'
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```
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We already know a similar thing -- square brackets. They allow to choose between multiple character, for instance `pattern:gr[ae]y` matches `match:gray` or `match:grey`.
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Alternation works not on a character level, but on expression level. A regexp `pattern:A|B|C` means one of expressions `A`, `B` or `C`.
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For instance:
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- `pattern:gr(a|e)y` means exactly the same as `pattern:gr[ae]y`.
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- `pattern:gra|ey` means "gra" or "ey".
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To separate a part of the pattern for alternation we usually enclose it in parentheses, like this: `pattern:before(XXX|YYY)after`.
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## Regexp for time
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In previous chapters there was a task to build a regexp for searching time in the form `hh:mm`, for instance `12:00`. But a simple `pattern:\d\d:\d\d` is too vague. It accepts `25:99` as the time.
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How can we make a better one?
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We can apply more careful matching:
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- The first digit must be `0` or `1` followed by any digit.
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- Or `2` followed by `pattern:[0-3]`
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As a regexp: `pattern:[01]\d|2[0-3]`.
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Then we can add a colon and the minutes part.
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The minutes must be from `0` to `59`, in the regexp language that means the first digit `pattern:[0-5]` followed by any other digit `\d`.
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Let's glue them together into the pattern: `pattern:[01]\d|2[0-3]:[0-5]\d`.
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We're almost done, but there's a problem. The alternation `|` is between the `pattern:[01]\d` and `pattern:2[0-3]:[0-5]\d`. That's wrong, because it will match either the left or the right pattern:
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```js run
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let reg = /[01]\d|2[0-3]:[0-5]\d/g;
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alert("12".match(reg)); // 12 (matched [01]\d)
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```
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That's rather obvious, but still an often mistake when starting to work with regular expressions.
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We need to add parentheses to apply alternation exactly to hours: `[01]\d` OR `2[0-3]`.
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The correct variant:
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```js run
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let reg = /([01]\d|2[0-3]):[0-5]\d/g;
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alert("00:00 10:10 23:59 25:99 1:2".match(reg)); // 00:00,10:10,23:59
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```
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