Struct regex::RegexBuilder

source ·
pub struct RegexBuilder(_);
Expand description

A configurable builder for a regular expression.

A builder can be used to configure how the regex is built, for example, by setting the default flags (which can be overridden in the expression itself) or setting various limits.


Create a new regular expression builder with the given pattern.

If the pattern is invalid, then an error will be returned when build is called.

Consume the builder and compile the regular expression.

Note that calling as_str on the resulting Regex will produce the pattern given to new verbatim. Notably, it will not incorporate any of the flags set on this builder.

Set the value for the case insensitive (i) flag.

When enabled, letters in the pattern will match both upper case and lower case variants.

Set the value for the multi-line matching (m) flag.

When enabled, ^ matches the beginning of lines and $ matches the end of lines.

By default, they match beginning/end of the input.

Set the value for the any character (s) flag, where in . matches anything when s is set and matches anything except for new line when it is not set (the default).

N.B. “matches anything” means “any byte” when Unicode is disabled and means “any valid UTF-8 encoding of any Unicode scalar value” when Unicode is enabled.

Set the value for the greedy swap (U) flag.

When enabled, a pattern like a* is lazy (tries to find shortest match) and a*? is greedy (tries to find longest match).

By default, a* is greedy and a*? is lazy.

Set the value for the ignore whitespace (x) flag.

When enabled, whitespace such as new lines and spaces will be ignored between expressions of the pattern, and # can be used to start a comment until the next new line.

Set the value for the Unicode (u) flag.

Enabled by default. When disabled, character classes such as \w only match ASCII word characters instead of all Unicode word characters.

Whether to support octal syntax or not.

Octal syntax is a little-known way of uttering Unicode codepoints in a regular expression. For example, a, \x61, \u0061 and \141 are all equivalent regular expressions, where the last example shows octal syntax.

While supporting octal syntax isn’t in and of itself a problem, it does make good error messages harder. That is, in PCRE based regex engines, syntax like \0 invokes a backreference, which is explicitly unsupported in Rust’s regex engine. However, many users expect it to be supported. Therefore, when octal support is disabled, the error message will explicitly mention that backreferences aren’t supported.

Octal syntax is disabled by default.

Set the approximate size limit of the compiled regular expression.

This roughly corresponds to the number of bytes occupied by a single compiled program. If the program exceeds this number, then a compilation error is returned.

Set the approximate size of the cache used by the DFA.

This roughly corresponds to the number of bytes that the DFA will use while searching.

Note that this is a per thread limit. There is no way to set a global limit. In particular, if a regex is used from multiple threads simultaneously, then each thread may use up to the number of bytes specified here.

Set the nesting limit for this parser.

The nesting limit controls how deep the abstract syntax tree is allowed to be. If the AST exceeds the given limit (e.g., with too many nested groups), then an error is returned by the parser.

The purpose of this limit is to act as a heuristic to prevent stack overflow for consumers that do structural induction on an Ast using explicit recursion. While this crate never does this (instead using constant stack space and moving the call stack to the heap), other crates may.

This limit is not checked until the entire Ast is parsed. Therefore, if callers want to put a limit on the amount of heap space used, then they should impose a limit on the length, in bytes, of the concrete pattern string. In particular, this is viable since this parser implementation will limit itself to heap space proportional to the length of the pattern string.

Note that a nest limit of 0 will return a nest limit error for most patterns but not all. For example, a nest limit of 0 permits a but not ab, since ab requires a concatenation, which results in a nest depth of 1. In general, a nest limit is not something that manifests in an obvious way in the concrete syntax, therefore, it should not be used in a granular way.

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