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StringStartsQ

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

StringStartsQ["string", patt]

Gives True if a prefix of "string" matches the string expression patt, and False otherwise.

StringStartsQ["string", {p1, p2, ...}]

Gives True if a prefix matches any of the pi.

StringStartsQ[{s1, s2, ...}, patt]

Gives the list of results for each of the si.

StringStartsQ[patt]

Represents an operator form that can be applied to a string. Equivalent to StringContainsQ["string", StartOfString ~~ patt]. Options: IgnoreCase -> True treats upper/lowercase as equivalent.

Examples (7)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (4)

In[1]:= StringStartsQ["commit", "co"]
Out[1]= True

In[2]:= StringStartsQ["commit", "om"]
Out[2]= False

In[3]:= StringStartsQ[{"apple", "banana"}, "a"]
Out[3]= {True, False}

In[4]:= StringStartsQ["a123", LetterCharacter ~~ DigitCharacter ..]
Out[4]= True

Applications (3)

A matching prefix

In[5]:= StringStartsQ["https://x", "http"]
Out[5]= True

No match

In[6]:= StringStartsQ["ftp://x", "http"]
Out[6]= False

Threads over a list

In[7]:= StringStartsQ[{"abc", "xyz"}, "a"]
Out[7]= {True, False}

Implementation notes

Algorithm. StringStartsQ is the SQ_STARTS face of the shared sq_dispatch core. Rather than thread a new anchor mode through the shared rule builder, it synthesises a temporary StringExpression[StartOfString, patt] and hands that unanchored to regex_rules_build_ex; the translator renders StartOfString as \A, an absolute anchor that pins the match to offset 0 even in an unanchored search. The translator's group_join wraps every child in (?:...), so an alternation-bearing pattern anchors correctly as (?:(?:\A)(?:a|b)) rather than the broken \Aa|b. Then the shared sq_match early-exits on the first hit.

Data structures. The synthesised wrapper Expr ā€” which must outlive the rule set, since RegexRule.lhs borrows into it ā€” plus the RegexRule array (whole-match pair only).

Complexity / limits. Equivalent to StringContainsQ[s, StartOfString ~~ patt]. IgnoreCase option; curried operator form StringStartsQ[patt]. A non-string subject leaves the call unevaluated; byte semantics.

Attributes: Protected.

References

Notes & additional examples

Notes

StringStartsQ[s, patt] is equivalent to StringContainsQ[s, StartOfString ~~ patt]: it anchors the pattern to the start of the string by wrapping it as StringExpression[StartOfString, patt] before matching.

It shares the predicate core with StringContainsQ, StringFreeQ, and StringEndsQ, takes an IgnoreCase option, and offers the curried operator form StringStartsQ[patt].